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

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, 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

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

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, reflects, respects, safe-sex, safe sex, same-sex, starplex, subjects, supaplex, supermex, suspects, teleflex, timeplex, tokyo mx, videotex, wall-tex, wrynecks, zanaflex

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

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

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

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

13 letters:
between decks, cinematheques, double-checks, double-duplex, freestyle bmx, single-duplex, tesla model x

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

15 letters:
final fantasy x, unsimulated sex

16 letters:
extramarital sex, tracey lambrechs

17 letters:
tyrannosaurus rex

18 letters:
american history x

19 letters:
non-penetrative sex

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

5 letters:
steps

7 letters:
accepts, affects, defects, directs

8 letters:
collects, connects, protects

9 letters:
sea steps, suspect's

11 letters:
macronectes

12 letters:
batrachoseps

13 letters:
last respects

15 letters:
flight of steps

16 letters:
personal effects, in some respects

18 letters:
pseudopleuronectes