4 letters:
buel, buhl, cool, dual, dule, fool, fuel, juel, jule, kool, mule, pool, poul, raul, reul, ruel, rule, tool, tuel, yule

5 letters:
-cule, abdul, adule, babul, baiul, beule, boole, boule, brule, buehl, capul, coole, crool, cruel, deuil, dhool, dhule, doole, dreul, drool, duell, ghoul, goole, grool, hoole, huile, jewel, joule, keule, khul', kreul, muehl, noule, pecul, poole, poule, rahul, raoul, reule, rhule, roule, ruhul, saoul, scool, shool, shoul, shule, skool, smule, spool, spule, stool, thule, toole, truel, tuell, tuyle, utsul, vipul, yeule, youll, yuill, zeuhl

6 letters:
alcool, as-30l, babool, baoule, befool, boulle, bruehl, cerule, choule, curule, dobule, ethule, ferule, frioul, g.soul, gueule, gurule, hyrule, infule, insoul, jambul, kaboul, kagoul, lemuel, lobule, locule, lucule, luelle, macule, mccool, muelle, nabeul, nepool, nucule, omtool, oscule, perule, pilule, recule, retool, schule, scrool, shmuel, tooele, tuille, uncool, unfool, vocule, who'll, you'll, yuille

7 letters:
aktuell, ampoule, arghool, barbule, bascule, bouille, bulbule, cagoule, caracul, carpool, ciboule, corcule, damfool, drogoul, edicule, ethtool, fanduel, ferrule, festool, feuille, fistule, guayule, hangeul, jambool, kagoule, karakul, kurnool, laycool, makhoul, maqbool, medjool, misrule, o'toole, outfool, pethuel, pistool, precool, pustule, rantoul, rrdtool, saccule, sandeul, schuele, sertule, sporule, sproule, sproull, spruell, spruill, texpool, tilleul, tomfool, unspool, valvule, vanpool, zebrule

8 letters:
aedicule, agrifuel, air-cool, amygdule, b-school, barstool, blastule, canicule, car-pool, cesspool, chacmool, chamisul, chevreul, clausule, claypool, corneule, coronule, cryptool, deadpool, deepcool, deschool, digitule, dindigul, dzhambul, entitule, exajoule, exiftool, floccule, floscule, gag rule, genepool, girlpool, glenpool, gwenpool, heeltool, intitule, ipsambul, istanbul, j-school, kab-500l, linalool, lodicule, lovefool, metarule, molecule, monocule, neekolul, opercule, opuscule, overrule, pedicule, plantule, radebeul, radicule, reticule, ridicule, sesspool, silicule, spherule, spruiell, tidepool, tim pool, top-tool, traboule, ullapool, unbefool

9 letters:
arbuscule, audiotool, blackpool, bloemkool, campstool, caroncule, chondrule, claypoole, cysticule, dromgoole, edge tool, faldstool, farigoule, fascicule, follicule, footstool, foot rule, gameduell, glomerule, graticule, grembiule, hand tool, hierodule, home-rule, home rule, intercool, kab-1500l, karnivool, kilojoule, lawn tool, lenticule, liverpool, machinule, majuscule, megajoule, minuscule, monticule, motorpool, multitool, nanojoule, outschool, parannoul, pedascule, pellicule, petajoule, petiolule, picojoule, piquepoul, planetule, playskool, poeticule, pontypool, preschool, propagule, ramuscule, rookstool, self-rule, steenkool, stepstool, supercool, teurgoule, tire tool, toadstool, ulcuscule, undercool, vestibule, welshpool, whirlpool, wurbagool

10 letters:
animalcule, april fool, board rule, boerenkool, canalicule, closestool, corpuscule, crepuscule, cuttystool, fenestrule, freedstool, frithstool, girlschool, gribouille, hartlepool, highschool, homeschool, labor pool, millijoule, minischool, motor pool, multi-tool, old-school, plastidule, playschool, plumb rule, poiseuille, power tool, pre-school, realschule, slide rule, somnambule, spinnerule, step stool, tinkertool, vanderpool, vanterpool

11 letters:
afterschool, biomolecule, close-stool, creepschool, cutty stool, dame-school, disseminule, garden tool, golden rule, ground rule, groupuscule, high-school, home-school, interschool, music stool, piano stool, that you'll, tokyo ghoul, wading pool, warrnambool

12 letters:
after-school, amuse-gueule, anonymuncule, apostolopoul, bellefeuille, berufsschule, bishop-stool, conciliabule, cutting tool, endoplastule, george boole, hearsay rule, hemispherule, languagetool, les claypool, machine tool, millefeuille, nonzero-rule, numbers pool, portefeuille, semifloscule, shaping tool, work-to-rule, working rule

13 letters:
chevrefeuille, crazysexycool, ducking-stool, ducking stool, gram-molecule, gram molecule, intellektuell, macromolecule, majority rule, milking stool, mille-feuille, out-of-school, peter o'toole, public-school, quatrefeuille, snipping tool, supermolecule, supramolecule, swimming-pool, swimming pool

15 letters:
annette o'toole, cyril abiteboul, dipole molecule, linguistic rule, lord of misrule

16 letters:
carpenter's rule, entrenching tool, grammatical rule, protein molecule

17 letters:
christopher poole, exclusionary rule

18 letters:
best evidence rule

25 letters:
korea under japanese rule

26 letters:
olympic-size swimming pool