3 letters:
ack, akc, bak, cac, dac, dak, fac, lac, lak, mac, mak, pac, sac, tac, yak, zak

4 letters:
back, flak, hack, jack, lack, mack, pack, rack, sack, tack, wack, yack

5 letters:
aback, anzac, black, brack, clack, crack, eniac, flack, gopac, ileac, knack, kodak, lilac, quack, shack, slack, smack, snack, stack, track, whack, wrack

6 letters:
anorak, attack, balzac, calpac, chirac, claque, cognac, compaq, hijack, ipecac, kodiak, maniac, plaque, prozac, ricrac, sacque, tarmac, tictac, unpack, zodiac

7 letters:
air sac, almanac, buyback, cardiac, carjack, cutback, dieback, elegiac, fatback, finback, go back, gum-lac, hatrack, hogback, hopsack, jacques, outback, payback, pontiac, ransack, redback, setback, shellac, skyjack, tieback, tintack

8 letters:
ammoniac, amnesiac, backpack, bareback, blowback, bootjack, brainiac, cadillac, callback, clawback, coatrack, comeback, creashak, cut back, demoniac, die back, drawback, etodolac, fallback, fastback, feedback, fireback, flapjack, fullback, get back, gimcrack, grayback, gripsack, halfback, hardback, haystack, highjack, holdback, humpback, ice pack, kickback, knapsack, man jack, mossback, packsack, pay back, playback, pullback, put back, rollback, roorback, rucksack, sad sack, seed lac, set back, simoniac, sit back, six-pack, skipjack, ski rack, slapjack, snowpack, softback, sulindac, swayback, tailback, talkback, tear sac, ticktack, tie rack, tie tack, wingback, win back, woolpack, yolk sac

9 letters:
amberjack, antiblack, applejack, backtrack, barmbrack, beat back, blackjack, blue jack, bomb rack, bootblack, box kodak, bushwhack, call-back, call back, cheapjack, come back, crookback, crossjack, date back, dish rack, draw back, drop back, face pack, fall back, feed back, flashback, give back, greenback, gunnysack, hang back, hark back, hatchback, hitchrack, hold back, horseback, huckaback, hunchback, ink-black, insomniac, jet-black, jet black, keep back, kick back, laid-back, lampblack, lean back, leaseback, look back, merrimack, move back, paperback, paranoiac, pickaback, piggyback, pipe rack, play back, pull back, push back, quillback, racetrack, razorback, rear back, roof rack, sea wrack, send back, shoeblack, sidetrack, slingback, spore sac, stick lac, sweptback, take back, thornback, throwback, thumbtack, toastrack, toe crack, toss back, touchback, trim back, turn back, wisecrack, wolf pack

10 letters:
ahorseback, air attack, blue-black, bring back, canvasback, carry back, cart track, cheap-jack, choke back, coal-black, coal black, cornerback, cul-de-sac, cul de sac, data track, dirt track, dress rack, drive back, fight back, flash back, force back, garnet lac, gunny sack, hackmatack, half-track, half track, inky-black, knickknack, knock back, lumberjack, monomaniac, music rack, natterjack, phone jack, pig-a-back, plate rack, power pack, pyromaniac, saddleback, sand crack, screw jack, shoe black, silverback, smokestack, sonora lac, soot black, soundtrack, spice rack, spur track, stand back, supplejack, switchback, take aback, think back, throw back, towel rack, turtleback, union jack, water back

11 letters:
biofeedback, bounce back, bubble pack, bumper jack, carpet tack, ceding back, click-clack, coming back, crackerjack, culs-de-sac, diamondback, dipsomaniac, double back, freddie mac, grass wrack, hollow-back, ivory black, ladder-back, leatherback, leatherjack, lobsterback, paying back, pericardiac, pitch-black, pitch black, prickleback, quarterback, shrink back, steeplejack, stickleback, welsh black, yellow jack

12 letters:
back-to-back, bibliomaniac, bicycle rack, bladderwrack, blister pack, carbon black, chimneystack, cinder track, common lilac, heart attack, hit the sack, inside track, jumping jack, luggage rack, megalomaniac, nymphomaniac, off-the-rack, panic attack, running back, under attack, whisker jack

13 letters:
asthma attack, counterattack, dental plaque, dressing sack, fatigue crack, fishing smack, from way back, ground attack, high-low-jack, literary hack, magazine rack, persian lilac, quarter crack, stock buyback

14 letters:
anxiety attack, clickety-clack, japanese lilac, plan of attack, platinum black, political hack, railroad track, ride horseback, serrated wrack, telephone jack

15 letters:
angle of attack, himalayan lilac, hungarian lilac, service cutback, streetcar track, surprise attack

16 letters:
bacterial plaque, break one's back

17 letters:
negative feedback, positive feedback, warning of attack

18 letters:
sign of the zodiac

19 letters:
diversionary attack, japanese tree lilac

20 letters:
off the beaten track