3 letters:
act

4 letters:
fact, pact, tact

5 letters:
achte, adact, anact, bract, coact, deact, enact, epact, exact, exakt, fract, gackt, jacht, lact-, nackt, pacht, packt, pacte, rakht, react, takht, tract, trakt, zacht

6 letters:
backed, bracht, dachte, didact, ex'act, fracht, galact, gedact, gracht, hacked, imfact, impact, intact, jacked, kracht, lacked, olfact, outact, packed, peract, pracht, preact, racked, re-act, redact, rofact, sacked, stacte, subact, tacked, tracht, unfact, wacked, yacked, yakked

7 letters:
-backed, abreact, anteact, attract, auftakt, biofact, blacked, brachte, clacked, compact, contact, cracked, detract, ecofact, edifact, en-tact, extract, flacked, galact-, gedacht, genpact, geofact, incoact, inexact, infract, in fact, knacked, kompakt, kontakt, medfact, one-act, playact, postact, quacked, qvacked, reenact, refract, retract, sanakht, schacht, sex act, shacked, slacked, smacked, snacked, stacked, trachte, tracked, unexact, whacked, wracked

8 letters:
abstract, acetract, antefact, artefact, artifact, attacked, autofact, benefact, betracht, cateract, cei-pact, cextract, confract, contract, diffract, distract, entracte, gebracht, hijacked, interact, kontakte, play-act, postfact, protract, re-enact, repacked, retroact, riot act, rotaract, schlacht, subtract, tadelakt, tocktact, transact, unbacked, underact, unhacked, unpacked

9 letters:
carjacked, eintracht, jampacked, medivaced, overreact, prepacked, ransacked, recontact, skyjacked, substract, tarmacked, tesseract, unslacked, untracked, ventifact, vkontakte

10 letters:
autodidact, backpacked, barebacked, car-jacked, cataphract, contrafact, counteract, enviropact, gobsmacked, hardbacked, highjacked, humpbacked, jam-packed, low-backed, medivacked, mossbacked, politifact, pontefract, precontact, retrofract, rollbacked, shadowpact, sister act, speech act, spiritpact, swaybacked, underreact

11 letters:
air-slacked, backtracked, bataireacht, blackjacked, bushwhacked, chain-react, crookbacked, cross-react, eye contact, finchbacked, fly contact, fullcontact, high-backed, high-impact, hunchbacked, lute-backed, nerve tract, noncontract, optic tract, paperbacked, piggybacked, precontract, semicompact, sidetracked, special act, subcontract, sway-backed, thumbtacked, wisecracked

12 letters:
asset-backed, bunch-backed, camel-backed, close-packed, cola extract, enabling act, fast-tracked, fluidextract, full-contact, half-cracked, half-tracked, non-contract, razor-backed, roach-backed, round-backed, saddlebacked, semiabstract, spiny-backed, stiff-backed, suicide pact, switchbacked, well-stacked

13 letters:
action-packed, broken-backed, fraud in fact, gabriel macht, huckle-backed, ladder-backed, lemon extract, oral contract, quarterbacked, saddle-backed, semi-abstract, urinary tract, vacuum-packed

14 letters:
almond extract, genshin impact, labor contract, lobster-backed, matter-of-fact, matter of fact, quasi-contract, quasi contract, smart contract

15 letters:
counterattacked, digestive tract, finding of fact, legislative act, mortgage-backed, output contract, social contract, straight-backed, vanilla extract

16 letters:
alimentary tract, attorney-in-fact, contrary to fact, futures contract, in point of fact, question of fact, service contract, special contract

17 letters:
accomplished fact, aleatory contract, gambling contract, marriage contract, purchase contract, respiratory tract

18 letters:
bilateral contract, breach of contract, electrical contact, term of a contract

19 letters:
affordable care act, as a matter of fact, caterpillar-tracked, employment contract, unilateral contract

20 letters:
conditional contract, create by mental act

21 letters:
requirements contract

22 letters:
gastrointestinal tract

24 letters:
accessory after the fact

25 letters:
accessory before the fact

34 letters:
occupational safety and health act