1 letter:
x

2 letters:
ex, x.

3 letters:
aix, atx, cmx, dex, hex, jex, lex, nex, rex, sex, tex, vex

4 letters:
amex, apex, flex, gatx, next, onex, plex, visx

5 letters:
-plex, aflex, alecs, altex, ampex, annex, apecs, apmex, artex, asecs, astex, autex, avtex, becks, bomex, camex, capex, carex, cedex, celtx, cemex, cimex, codex, comex, corex, cplex, datex, ddx3x, dec's, decks, defex, desex, devex, digex, eckes, endex, entex, eques, execs, fecks, fedex, finex, firex, flex-, futex, gitex, govpx, hapex, inmex, inpex, insex, intex, katex, kecks, kotex, latex, lipex, logex, lzexe, mapex, memex, metex, mutex, n.mex, necks, neorx, nomex, nuttx, nymex, nynex, opecs, parex, pechs, pecks, pemex, pewex, pipex, podex, ramex, recks, redex, remex, retex, romex, rtecs, sarex, sechs, secks, sekes, sofex, specs, sphex, sysex, tarex, techs, telex, treks, truex, typex, unsex, videx, winmx, xetex, sects

6 letters:
alopex, annexe, apec's, apotex, aramex, auspex, aztecs, beck's, berlex, bibtex, binhex, bitmex, caltex, carmex, centex, checks, codecs, colmex, compex, connex, convex, crownx, cusecs, czechs, daplex, de-sex, deflex, dentex, diplex, duplex, ejects, flecks, frutex, gentex, heck's, hiprex, imbrex, implex, inflex, invecs, keflex, klecks, knipex, konvex, libvpx, luatex, macosx, meckes, mi-sex, miktex, navtex, opec's, orbitx, pdftex, peck's, pictex, pretex, q-flex, qintex, rebecs, rednex, rejctx, scitex, sensex, sharex, shek's, silvex, snecks, specks, superx, suplex, sveccs, syntex, tech's, teck's, telmex, texmex, thraex, torpex, tremex, tridex, trihex, umarex, unisex, vert.x, vertex, winhex, wrecks, yu-mex, elects, erects, hechts, rechts, sect's, spects

7 letters:
adaplex, andratx, aniplex, antapex, apoplex, artifex, aruspex, banamex, bedecks, blech's, bowflex, celotex, cheques, cmd.exe, complex, comtrex, convexe, copecks, craptex, cryptex, datarex, deus ex, directx, dvb-s2x, effects, endosex, equitex, erosstx, exo-cbx, expects, flexatx, frontex, fss ecs, funplex, goretex, graflex, gtech's, homosex, humidex, hyponex, inditex, jadetex, klaypex, komplex, kopecks, liga mx, medarex, memorex, mirapex, mixtecs, narthex, natec's, nobec's, noindex, norflex, notimex, novamex, objects, oraflex, panorex, parsecs, paychex, perplex, perspex, playtex, pokedex, proflex, railtex, re-flex, re/flex, rebecks, rejects, rolodex, screenx, sheck's, simplex, sonic x, startex, subutex, supplex, synflex, sysplex, teletex, tenrecs, tex-mex, tmp-smx, toltecs, transex, trintex, triplex, twistex, vieques, waifu2x, zebecks, zoladex, advects, affects, aspects, bisects, defects, dejects, detects, directs, elect's, hecht's, infects, injects, insects, resects, selects, wecht's

8 letters:
aeroflex, ametek's, arimidex, armtek's, atriplex, audiotex, biconvex, biotechs, cineplex, clarinex, complexe, computex, cybersex, dissects, echoplex, fanech's, fantomex, faslodex, ferromex, fourplex, gore-tex, haruspex, have sex, henpecks, intel xe, interrex, intersex, intertex, iphone x, keepassx, mediplex, megaplex, memeplex, memtec's, methanex, mindflex, mineplex, mini-itx, mini-stx, multidex, nano-itx, nasa-tlx, oeko-tex, oldcodex, oral sex, phanteks, photodex, pico-itx, portec's, projects, quadplex, quebec's, quinidex, rechecks, rednecks, respects, safe-sex, safe sex, same-sex, starplex, subjects, supaplex, supermex, suspects, teleflex, timeplex, tokyo mx, videotex, wall-tex, wrynecks, zanaflex, analects, collects, confects, connects, corrects, deflects, dialects, inflects, insect's, inspects, neglects, pandects, perfects, prefects, prelects, protects, reelects, trajects, trisects

9 letters:
5-simplex, adaptec's, ams-latex, citeseerx, cromlechs, cytotechs, decomplex, dorsiflex, eyetech's, foredecks, goatse.cx, honda nsx, hyperflex, leicaflex, lil nas x, longnecks, malcolm x, metroplex, middlesex, movieplex, multiplex, paychecks, phone-sex, pillowtex, retroflex, suffrutex, tussionex, unperplex, ventritex, wedtech's, complects, deselects, diadectes, idiolects, projekcts, re-elects, redirects, suspect's, suspects', transects, vivisects

10 letters:
adaptaplex, afterdecks, ark-la-tex, belowdecks, cache-sexe, charli xcx, circumflex, contraplex, crooknecks, eurochecks, euroduplex, googleplex, googolplex, goosenecks, intellects, intersects, log-convex, osiris-rex, prentnieks, quadraplex, quadruplex, rolleiflex, roughnecks, single-sex, solar apex, stratoflex, sugammadex, symantec's, westtech's, architects, disaffects, disinfects, genuflects, interjects, introjects, misdirects, recollects, reconnects, resurrects, webobjects

11 letters:
ameritech's, begonia rex, bottlenecks, circonflexe, crosschecks, demultiplex, full-duplex, gendrisek's, genentech's, glendimplex, half-duplex, megaparsecs, planoconvex, rubbernecks, spot-checks, steinbeck's, terminatexe, turtlenecks, architect's, disconnects, disrespects, introspects, macronectes, pluperfects, retrospects

12 letters:
discotheques, g-quadruplex, gaztelugatxe, heteroduplex, hudson & rex, kaisertech's, leathernecks, leibniz-keks, nissan 240sx, plano-convex, quarterdecks, aftereffects, overprotects, side-effects, trichodectes

13 letters:
between decks, cinematheques, double-checks, double-duplex, freestyle bmx, single-duplex, tesla model x, after-effects, interconnects, last respects

14 letters:
concavo-convex, convexo-convex, herpes simplex, masters of sex

15 letters:
final fantasy x, unsimulated sex, businessobjects

16 letters:
extramarital sex, tracey lambrechs, in some respects, personal effects

17 letters:
tyrannosaurus rex

18 letters:
american history x, pseudopleuronectes, the usual suspects

19 letters:
non-penetrative sex, adobe after effects

20 letters:
cigarettes after sex, regression of y on x

5 letters:
steps

7 letters:
accepts

9 letters:
sea steps

12 letters:
batrachoseps

15 letters:
flight of steps