4 letters:
sect

5 letters:
-sect, becht, dsect, eject, elect, erect, exect, fecht, hecht, hect-, jtekt, pecht, recht, recte, secte, seked, sekte, spect, wecht

6 letters:
advect, affect, allect, arrect, aspect, becked, bisect, brecht, cilect, corect, defect, defekt, deject, deregt, detect, direct, effect, enfect, expect, exsect, fecked, humect, infect, inject, insect, insekt, invect, kecked, kinect, knecht, necked, object, obtect, pecked, precht, recked, refect, reject, resect, select, specht, speckt, unecht, utecht, vpec-t

7 letters:
-decked, -necked, 5select, analect, atrecht, bedeckt, checked, chek-dt, collect, confect, conject, connect, contect, convect, correct, deflect, despect, detrect, dialect, dialekt, directe, dissect, flecked, inflect, insecte, inspect, konfekt, neglect, pandect, perfect, perfekt, porrect, prefect, prelect, project, protect, provect, reelect, reerect, reflect, respect, snecked, specked, subject, subnect, suspect, suspekt, traject, tranect, trekked, trisect, unerect, utrecht, wrecked

8 letters:
acrolect, albrecht, artilect, aufrecht, basilect, bedecked, codirect, collecte, complect, deselect, dialecte, entrecte, hemisect, idiolect, indirect, mesolect, nonelect, preelect, preerect, pro-ject, re-elect, re-erect, redirect, reinfect, reselect, schlecht, suberect, topolect, transect, undirect, unselect, v-necked, venesect, vivisect

9 letters:
architect, architekt, aufgeregt, caverject, disaffect, disinfect, dordrecht, ethnolect, genuflect, hambrecht, henpecked, incorrect, intellect, interject, intersect, intertect, in effect, jetdirect, leberecht, misaffect, misdirect, multisect, nondirect, parkitect, predilect, preselect, rechecked, recollect, reconnect, rednecked, reinspect, resurrect, rupprecht, semierect, senkrecht, sociolect, unchecked, uncorrect, unperfect, unprotect, unrespect, unsuspect, wrynecked

10 letters:
amish sect, anderlecht, architecte, barenecked, bitconnect, bullnecked, by-respect, disconnect, disrespect, flyspecked, genderlect, introspect, miscorrect, moordrecht, non-direct, overaffect, pluperfect, quadrisect, re-collect, retrospect, semi-erect, subdialect, wax insect, wu dialect

11 letters:
aftereffect, autocorrect, circumflect, circumspect, freshdirect, half-decked, introselect, leaf insect, microinject, min dialect, openconnect, oranjegekte, overcorrect, overprotect, photoeffect, pinnatisect, privatrecht, quickselect, side-effect, side effect, skin effect, starchitect, superinfect, take effect, teleconnect, tigerdirect, yue dialect

12 letters:
after-effect, bottlenecked, crosschecked, eyebuydirect, hypercorrect, interconnect, misrecollect, note-perfect, over-correct, past perfect, quantconnect, rape suspect, rubbernecked, scale insect, schildknecht, schiltknecht, self-correct, self-neglect, self-respect, sound effect, spot-checked, stage direct, stage effect, stick insect, superinspect, transdialect, transperfect, turtlenecked, urheberrecht, word-perfect

13 letters:
1-800-collect, bipinnatisect, child neglect, cross-connect, dopplereffekt, double-decked, droste effect, dr disrespect, incircumspect, law of effect, liege subject, preterperfect, sciencedirect, short subject, uncircumspect, visual aspect, write-protect

14 letters:
counterchecked, doppler effect, double-checked, future perfect, letter-perfect, murder suspect, special effect, spittle insect, unwind-protect

15 letters:
beijing dialect, flemish dialect, in that respect, picture-perfect, present perfect, president-elect, preterimperfect, robbery suspect, ubisoft connect, willful neglect, wreckx-n-effect

16 letters:
coattails effect, mandarin dialect, preterpluperfect, shanghai dialect, with all respect

17 letters:
cantonese dialect, greenhouse effect

18 letters:
progressive aspect

19 letters:
landscape architect

20 letters:
triboelectric effect

4 letters:
kept, sept, wept

5 letters:
slept, adept, swept, inept

6 letters:
unkept, except, accept

7 letters:
stepped, unswept

8 letters:
transept

9 letters:
intercept, backswept, well-kept

10 letters:
nympholept

13 letters:
tempest-swept