4 letters:
ater

5 letters:
-ator, aitor, bater, cater, dater, dator, gater, gator, hater, kater, later, maatr, mater, matre, nater, pater, patre, rater, sator, satre, tater, tator, vater, vator, waitr, yater

6 letters:
abater, abator, abatur, acater, baiter, clater, crater, epater, flater, frater, gaiter, gaytre, grater, hayter, krater, maatre, plater, prater, quatre, skater, slater, stater, stator, trator, viator, waiter

7 letters:
ablator, aviator, citator, claytor, cratere, creator, curator, debater, debatir, debator, decatur, dilater, dilator, donator, eighter, equater, equator, fixator, greater, gyrator, kreator, levator, ligator, locater, locator, misater, mutator, necator, negator, notator, novator, plaiter, potator, praytor, rapator, rebater, relater, relator, rotator, slayter, sleator, sopater, strater, traiter, traitor, updater, vacatur

8 letters:
actuator, adulator, agitator, akinator, aligator, allaiter, animator, ballater, barrater, bellator, collator, deflater, deflator, deviator, dictator, diktator, educator, elevator, emanator, embratur, emulator, epilator, evocator, expiator, fellator, hate her, hydrator, idolater, idolator, imitater, imitator, inflater, inflator, isolator, iterator, lemaitre, mandator, mediator, mercator, migrator, narrater, nitrator, obviator, operator, oxidator, ozonator, piscator, probator, pronator, pulsator, rabiator, radiator, regrater, regrator, saltator, schlater, serrator, testator, titrator, tronator, upstater, urinator, valuator, vanmatre, violator, zoolater

9 letters:
abdicator, abnegator, abrogator, activator, advocator, agadmator, agistator, alienator, allegator, alligator, allocator, allocatur, ambulator, amputator, annotator, arborator, archiater, arendator, arrogator, aspirator, await her, baconator, bloviator, castrater, castrator, cinerator, cogitator, conciator, constater, decimator, decorator, dedicator, defecator, delegator, deputator, desolator, detonator, dilatator, divinator, dominator, emendator, emigrator, escalator, escheator, estimator, evacuator, evaluator, excavator, excitator, expilator, fabulator, forfaiter, freighter, fumigator, generator, gladiator, glossator, graduator, grallator, habitator, hesitater, hesitator, hostgator, immolator, incubator, indagator, inhalator, innovator, insulator, intubator, inundator, irrigator, irritator, jaculator, jetavator, joculator, judicator, jugulator, kegerator, laminator, lapidator, levitator, linklater, literator, litigator, macerator, malaxator, medicator, meditator, mitigator, moderator, modulator, monolater, mutilator, navigator, nominator, nucleator, numerator, obligator, obturator, opiniator, ordinator, osculator, peculator, permeator, populator, punctator, regulator, renovator, resonator, revelator, rotavator, rotovator, ruminator, salivator, saturater, saturator, separator, seperator, simulator, sosipater, spoliator, supinator, tabulator, tailgater, tolerator, undulator, venerator, verilator

10 letters:
abominator, aggravator, alleviator, alternator, applicator, approbator, arbitrator, arpentator, associator, astrogator, astrolater, attenuator, bardolater, bay stater, buccinator, calculator, calibrater, calibrator, captivator, carbonator, carburator, carburetor, castigator, celebrater, celebrator, circulator, coagulator, combinator, commutator, companator, compilator, compotator, conjugator, conjurator, consolator, convocator, cooperator, corporator, correlator, corrugator, criminator, cultivator, cybernator, declamator, decollator, defalcator, defoliator, delineator, demarcator, denigrator, deprecator, depredator, desecrater, desecrator, designator, devastator, dissipater, dissipator, downstater, dumbwaiter, duplicator, ejaculator, elaborator, eliminator, elucidator, elutriator, enucleator, enumerator, enunciator, epitomator, eradicator, evaporator, examinator, excoriator, exonerator, expatiator, explicator, exsiccator, extenuator, extirpator, fabricator, fascinator, formulator, fornicator, frustrater, fulminator, germinator, gonkulator, governator, granulator, gurgitator, hagiolater, headwaiter, hibernator, humiliator, ice-skater, iconolater, imprecator, incarnator, inculcator, inoculator, insidiator, inspirator, instigator, integrator, irradiator, jew-baiter, kelvinator, lacrimator, landwaiter, legislator, liquidator, lubricator, machinator, masticater, masticator, meliorater, meliorator, methylator, motograter, negotiator, obfuscator, objurgator, officiator, originator, oscillator, ostentator, oxygenator, pdfcreator, penetrator, percolator, perforator, permulator, personator, pollinator, postulator, predicator, procreator, propagator, prostrator, punctuator, reanimator, replicator, repudiator, respirator, retaliator, rubricator, rusticator, sea slater, segregator, sequenator, speculator, stabilator, stimulater, stimulator, stipulator, straighter, subjugator, syncopator, syndicator, terminator, titillater, titillator, translater, translator, travelator, travolator, triturator, vacillator, ventilator, vindicator

11 letters:
abbreviator, aberuncator, accelerator, accumulator, adjudicator, adulterator, affreighter, alliterator, amalgamator, ameliorator, annihilator, anticipator, appreciator, archtraitor, arcovenator, arpeggiator, articulator, asphyxiator, asseverator, assimilator, auktionator, auscultator, bibliolater, brusselator, cachinnator, calorisator, calumniator, capitulator, centuriator, chlorinator, co-operator, cogenerator, commendator, commentator, compensator, compurgator, conciliator, confiscator, congregator, consecrater, consecrator, consummator, continuator, coordinator, deactivator, decapitator, decelerator, deliberator, demodulator, demonolater, denominator, denunciator, depopulator, depreciator, desalinator, dilapidator, dissertator, dumb-waiter, edulcorator, emancipator, emasculator, equivocator, eviscerator, exaggerator, exasperater, excelerator, excogitator, exhilarator, facilitator, felicitator, first-rater, flagellator, habilitator, head waiter, illuminator, illustrator, imperobator, impregnator, inaugurator, incinerator, infiltrator, inseminator, inspissator, instillator, insufflator, intimidator, invalidator, invigilator, invigorator, lachrymator, legitimator, manipulator, masturbator, metadate er, modificator, nomenclator, obliterator, opinionator, pacificator, perpetrator, perpetuator, perturbator, phabricator, pond-skater, postillator, potentiator, promulgator, protestator, purificator, racetraitor, re-animator, recuperater, recuperator, redecorator, regenerator, rejuvenator, remunerator, shad-waiter, speedskater, stellarator, stridulator, suffragator, sulphurator, summercater, supplicator, telestrator, telustrator, third-rater, totalisator, totalizator, trafficator, twerkulator, vasodilator, vaticinator, ventilatore, vinificator, vituperator, whizzinator, wine waiter, woman-hater, woman hater

12 letters:
accommodator, achillobator, adminstrator, agglomerator, amphitheater, amphitheatre, appropriator, approximator, assassinator, averruncator, bioindicator, co-ordinator, collaborator, commemorator, commiserator, communicator, concavenator, concentrator, confabulator, confederator, considerator, consolidator, contaminator, contemplator, corroborator, decorticator, degerminator, demonstrater, demonstrator, dephlegmator, determinator, disseminator, dissimulator, domesticator, dracovenator, encapsulator, expostulator, expropriator, exterminator, extrapolator, falsificator, fractionator, gesticulator, hypothecator, impropriator, improvisator, incarcerator, incorporator, incriminator, individuator, instilllator, interpolator, interrogator, investigator, justificator, lunar crater, mystificator, optoisolator, orchestrator, paraeducator, perambulator, peregrinator, pontificater, pontificator, precipitator, predominator, premeditator, proliferator, pronunciator, qualificator, ratiocinator, reciprocator, recriminator, rectificator, refrigerator, regurgitator, remonstrator, resuscitator, reverberator, sacrificator, scarificator, scintillator, second-rater, sequestrator, sermocinator, significator, stabilisator, subordinator, teleoperator, testificator, versificator, water skater, wheelabrator

13 letters:
28 days later, administrator, aerogenerator, authenticator, conglomerator, congratulator, defibrillator, dineobellator, disambiguator, discriminator, disintegrator, electroplater, figure-skater, food elevator, full radiator, indoctrinator, intermediator, interpellator, monochromator, multiplicator, nanogenerator, neverthelater, noncooperator, opensimulator, opto-isolator, osmoregulator, out-of-stater, poweranimator, predestinator, prefabricator, recapitulator, redintegrator, refridgerator, reinvigorator, roller-skater, somnambulator, sophisticator, transformator, wurstelprater

14 letters:
28 weeks later, astronavigator, bronchodilator, decontaminator, denitrificator, differentiator, disarticulator, excommunicator, experimentator, ichthyovenator, neuromodulator, owner-operator, procrastinator, prognosticater, prognosticator, room decorator, selling-plater, superfreighter, transliterator, turbogenerator, wind generator

15 letters:
circumnavigator, congress gaiter, flux applicator, genus alligator, hand calculator, house decorator, immunomodulator, linear operator, nash-kelvinator, neurostimulator, photocoagulator, pulse generator, pump-oxygenator, rotary actuator, sooner or later, telemanipulator, thermoregulator

16 letters:
chicxulub crater, company operator, engine-generator, flight simulator, freight elevator, growth regulator, hyperaccumulator, laplace operator, local oscillator, magnetic equator, micromanipulator, nitrovasodilator, service elevator, transilluminator, turboencabulator

17 letters:
celestial equator, chinese alligator, elevator operator, identity operator, immunopotentiator, pocket calculator, quartz oscillator, transubstantiator, voltage regulator, yandere simulator

18 letters:
american alligator, common denominator, crystal oscillator, darvaza gas crater, interior decorator, linear accelerator, newsstand operator, proton accelerator, sales demonstrator, telegraph operator, telephone operator, the great dictator

19 letters:
christophe lemaitre, counterdemonstrator, oxygen concentrator

20 letters:
electron accelerator, google authenticator, particle accelerator, private investigator, switchboard operator

21 letters:
electric refrigerator, heterodyne oscillator, induction accelerator, modulator-demodulator

22 letters:
academic administrator, principal investigator

23 letters:
electrostatic generator, random number generator

24 letters:
thermoelectric generator

26 letters:
microsoft flight simulator

28 letters:
microwave linear accelerator, tissue plasminogen activator

13 letters:
middlebreaker

14 letters:
mischief-maker

15 letters:
prestidigitator