7 letters:
oligist, ologist

8 letters:
oologist

9 letters:
apologist, biologist, ecologist, enologist, geologist, myologist, neologist, noologist, orologist, otologist, urologist, zoologist

10 letters:
aerologist, agrologist, algologist, apiologist, areologist, atmologist, axiologist, bryologist, bugologist, cetologist, cytologist, ethologist, etiologist, fetologist, gemologist, horologist, ideologist, indologist, mazologist, misologist, mixologist, monologist, mycologist, nomologist, nosologist, oenologist, oncologist, ontologist, pedologist, penologist, podologist, pomologist, pyrologist, rheologist, serologist, sexologist, sinologist, theologist, topologist, typologist, virologist, xenologist, zymologist

11 letters:
acarologist, aetiologist, agriologist, andrologist, angiologist, astrologist, audiologist, battologist, carpologist, cereologist, chirologist, cosmologist, ethnologist, etymologist, fungologist, garbologist, gemmologist, genealogist, hagiologist, herbologist, heroologist, hierologist, hippologist, histologist, hydrologist, hymnologist, hypnologist, iconologist, iridologist, limnologist, lithologist, mantologist, mariologist, metrologist, moirologist, museologist, myriologist, mythologist, necrologist, neurologist, ophiologist, osteologist, paleologist, pantologist, pathologist, patrologist, petrologist, phenologist, philologist, phonologist, photologist, phycologist, phytologist, psycologist, quinologist, radiologist, rhinologist, scatologist, semiologist, sociologist, symbologist, tautologist, tribologist, uranologist

12 letters:
archeologist, balneologist, bibliologist, cardiologist, choreologist, chronologist, conchologist, craniologist, creatologist, cryptologist, deltiologist, demonologist, dendrologist, deontologist, egyptologist, embryologist, entomologist, enzymologist, erpetologist, exobiologist, futurologist, gastrologist, glaciologist, glassologist, glottologist, graphologist, gynecologist, hematologist, hepatologist, immunologist, lexicologist, malacologist, meterologist, mineralogist, morphologist, musicologist, nematologist, nephrologist, numerologist, oceanologist, odontologist, oryctologist, palynologist, papyrologist, phrenologist, physiologist, psephologist, pseudologist, psychologist, saintologist, seismologist, selenologist, somatologist, speleologist, spermologist, synecologist, technologist, teratologist, teuthologist, toxicologist, trichologist, urbanologist, vermeologist

13 letters:
agrobiologist, agrostologist, arachnologist, archaeologist, assyriologist, bromatologist, campanologist, carcinologist, christologist, climatologist, cosmetologist, criminologist, cryobiologist, dermatologist, diabetologist, electrologist, entozoologist, eschatologist, galvanologist, gerontologist, gynaecologist, haematologist, heresiologist, herpetologist, ichthyologist, kinesiologist, laryngologist, malariologist, martyrologist, meteorologist, methodologist, myrmecologist, neonatologist, oil geologist, onomatologist, opthamologist, orchidologist, ornithologist, pantheologist, phraseologist, planetologist, primatologist, pteridologist, pulmonologist, reflexologist, scientologist, sovietologist, spelaeologist, stomatologist, syphilologist, tegestologist, terminologist, thanatologist, vaccinologist, venereologist, vexillologist, volcanologist, vulcanologist

14 letters:
anthropologist, astrobiologist, astrogeologist, bacteriologist, chiroptologist, dialectologist, dysteleologist, ecclesiologist, epidemiologist, epistemologist, fermentologist, hydrobiologist, hydrogeologist, kremlinologist, liturgiologist, microbiologist, neurobiologist, opthalmologist, palaetiologist, paleobiologist, paleoecologist, paleontologist, paleozoologist, parasitologist, perinatologist, pharmacologist, photobiologist, photogeologist, pneumatologist, protozoologist, radiobiologist, rheumatologist, sociobiologist, traumatologist, universologist

15 letters:
biotechnologist, chronobiologist, conchyliologist, crustaceologist, cryptozoologist, cytopathologist, ecotoxicologist, endocrinologist, geochronologist, geomorphologist, helminthologist, numismatologist, ophthalmologist, palaeontologist, phenomenologist, promorphologist, psychobiologist, roentgenologist

16 letters:
anesthesiologist, chiropterologist, cytotechnologist, histopathologist, nanotechnologist, neuro-oncologist, neuropathologist, neuroradiologist, otolaryngologist, paleopathologist, paleophytologist, parapsychologist, phytolithologist, phytopathologist

17 letters:
anaesthesiologist, electro-biologist, ethnomusicologist, lepidopterologist, neurophysiologist, neuropsychologist, psychopathologist

18 letters:
dendrochronologist, ethnomethodologist, gastroenterologist, hydrometeorologist, immunohematologist, paleoclimatologist, psychophysiologist, rhinolaryngologist

19 letters:
electrophysiologist, micropaleontologist, neuropharmacologist, palaeoclimatologist, paleoanthropologist, petroleum geologist

20 letters:
neuroendocrinologist, palaeoanthropologist, psychopharmacologist

21 letters:
clinical psychologist, otorhinolaryngologist, social anthropologist

7 letters:
holibut, opulent, oxidant

8 letters:
copyedit, botanist, optimist, colonist, dominant, monument, nominate, novelist, occupant, opposite, oscitant, populist

9 letters:
dramatist, competent, alchemist, economist, dogmatist, occultist, abominate, brominate, cognisant, cognizant, columnist, communist, condiment, confident, consonant, consulate, continent, emolument, obstinate, postulant, postulate, prominent, proselyte, proximate, somnolent

10 letters:
agronomist, monogamist, misogamist, bonduc nut, bon vivant, complement, consequent, consummate, flocculent, monopolist, protestant

11 letters:
optometrist, romanticist, antioxidant, approximate, concomitant, improvident, incognizant, incompetent, incontinent, predominant, predominate, subdominant, unconfident

12 letters:
proctologist, bibliopolist, proconsulate

14 letters:
self-confident, stop consonant

15 letters:
anthropophagite, fifth columnist, full complement, nasal consonant

16 letters:
dental consonant, gossip columnist

18 letters:
alveolar consonant, guttural consonant

19 letters:
affricate consonant, antarctic continent, newspaper columnist, voiceless consonant