1 letter:
i, y

2 letters:
ai, bi, by, cy, fi, hi, hy, i., ly, my, pi, sy, ty, vi, y.

3 letters:
aye, bae, buy, bye, cai, chi, cry, csi, dai, die, dry, dui, dwi, dye, eye, fae, fbi, fly, fry, fyi, guy, hye, kai, lai, lie, lye, mai, mei, nye, phi, pie, ply, pri, pry, psi, pye, rye, sai, shy, sky, sly, spy, sty, tai, thy, tie, tri, try, tye, vie, wai, why, wry, wye

4 letters:
alai, ally, awry, chai, defy, deny, frye, high, if i, july, kwai, nigh, rely, sigh, skye, spry, thai, wifi

5 letters:
alibi, and i, apply, assai, bahai, barye, belie, bligh, but i, dalai, debye, deci-, decry, deify, do by, dubai, edify, flyby, go by, hi-fi, imply, in my, kadai, kauai, khmer, lanai, rabbi, radii, reify, reply, retie, retry, salai, sinai, styli, thigh, unify, untie, wi-fi

6 letters:
adonai, aerify, alkali, alumni, annuli, aurify, basify, bigeye, bonsai, botfly, brunei, citify, codify, comply, cumuli, dayfly, descry, fly by, gadfly, gemini, get by, goodby, hereby, kichai, kowhai, lay-by, lenify, magpie, mayfly, minify, modify, mumbai, nazify, nearby, nilgai, notify, nuclei, occupy, ossify, outcry, pacify, pigsty, plaguy, potpie, purify, ramify, rarify, ratify, ripply, rubify, run by, salify, sawfly, sci-fi, sit by, supply, that i, typify, uglify, verify, vilify, vinify, vivify, what i, zip by

7 letters:
acetify, acidify, air-dry, alveoli, amplify, assegai, bad guy, beatify, bee fly, blowfly, bow tie, buckeye, bye-bye, calcify, calculi, carnify, certify, chennai, clarify, cockeye, come by, crucify, cup tie, damnify, dandify, dignify, drop by, dry fly, dulcify, falsify, far cry, firefly, fishfly, fortify, gallfly, glorify, good-by, goodbye, gratify, hokusai, horrify, jellify, justify, kam-tai, lac dye, let fly, lignify, liquefy, liquify, live by, lorelei, lullaby, magnify, metrify, mollify, molokai, mortify, mummify, mystify, necktie, nigrify, nitrify, nullify, nutrify, nylghai, osmanli, overbuy, overfly, pass by, pe-tsai, petrify, pinkeye, pinkify, pop fly, qualify, rectify, red-eye, red tai, reunify, run dry, russify, salsify, samurai, sandfly, satisfy, scarify, scorify, scrapie, sensify, sesqui-, shadfly, shoofly, signify, slip by, sockeye, songhai, specify, standby, stimuli, stupify, termini, terrify, testify, thereby, thurify, tie dye, uruguay, vat dye, verbify, versify, vitrify, walleye, war cry, wet fly, whereby

8 letters:
abide by, alderfly, alkalify, ammonify, antinomy, beautify, blackfly, blow-dry, blow fly, blue sky, bola tie, bone-dry, bone dry, c. p. i., catchfly, chagatai, chigetai, classify, denazify, detoxify, disunify, drip-dry, drive-by, emulsify, esterify, etherify, evil eye, exist by, f. b. i., fall guy, fast dye, fish fry, fly high, fructify, gentrify, glide by, good-bye, good guy, gramarye, greeneye, greenfly, hair dye, hand dye, horn fly, horsefly, housefly, humidify, identify, in reply, jai alai, lapidify, m. c. i., m. r. i., meat pie, mid-july, misapply, mordecai, multiply, musaceae, p. r. i., pork pie, prettify, quantify, revivify, rigidify, sand fly, saponify, selachii, shanghai, shoo fly, silicify, simplify, sky-high, slide by, snakefly, solidify, spin-dry, stand by, stick by, stir fry, stonefly, stratify, stultify, t. c. i., tent-fly, test fly, underlie, whisk by, whitefly, wild rye, wise guy

9 letters:
alder fly, antalkali, apple pie, black eye, black fly, black tai, black tie, butterfly, by-and-by, by and by, caddisfly, cat's eye, chondrify, crane fly, cross-eye, damselfly, decalcify, decertify, demulsify, demystify, denitrify, devitrify, diversify, dobsonfly, dragonfly, dziggetai, electrify, exemplify, fasciculi, flesh fly, frenchify, fruit fly, george vi, glass eye, goldeneye, havasupai, horse fly, indemnify, intensify, knee-high, limicolae, loasaceae, louse fly, magic eye, mince pie, passer-by, pecan pie, personify, pizza pie, preachify, preoccupy, rough-dry, scrape by, screw eye, slant-eye, small fry, speechify, spongefly, spray-dry, squeak by, stink fly, stone fly, syllabify, third eye, tough guy, travel by, well-nigh, white-eye, white tai, white tie

10 letters:
battle cry, bird's eye, bull's eye, by the bye, caddicefly, chalcidfly, complexify, declassify, disqualify, dissatisfy, dobson fly, double dye, eau de vie, freeze-dry, french fry, french rye, goggle-eye, gravimetry, jewish rye, kidney pie, lanternfly, mayacaceae, mind's eye, on the sly, oversupply, pineal eye, presignify, public eye, reclassify, robber fly, saccharify, simple eye, squash pie, squeeze by, string tie, tongue tie, tsetse fly, versailles, waist-high

11 letters:
aniline dye, ascidiaceae, bell magpie, breast-high, crossed eye, ephedraceae, harvest fly, hessian fly, hue and cry, impulse-buy, italian rye, junior high, lantern fly, misidentify, mount sinai, myxophyceae, private eye, pumpkin pie, red buckeye, rhubarb pie, saururaceae, senior high, shoofly pie, smilacaceae, spanish fly, styracaceae, vinegar fly, weather eye, windsor tie

12 letters:
blue walleye, caravanserai, central thai, compound eye, cupressaceae, cyanophyceae, davalliaceae, discocephali, divisible by, dominated by, dragon's eye, electric eye, geglossaceae, give the eye, hook and eye, horse botfly, human botfly, mackerel sky, money supply, northern spy, ohio buckeye, oleandraceae, oversimplify, papaveraceae, phaeophyceae, railroad tie, rallying cry, rhodophyceae, sheep botfly, sheep gadfly, southern tai, streamer fly, transmogrify, water supply

13 letters:
blueberry pie, captain bligh, chrysophyceae, dwarf buckeye, geoglossaceae, give it a try, interstratify, sacrifice fly, schizophyceae, sweet buckeye, william bligh

14 letters:
diatomophyceae, elasmobranchii, euglenophyceae, fourth of july, hempen necktie, indivisible by, in a pig's eye, menispermaceae, old school tie, red bird's eye, see eye to eye, shepherd's pie

15 letters:
american magpie, blink of an eye, canada wild rye, comma butterfly, european magpie, fluorescent dye, ophioglossaceae

16 letters:
boston cream pie, lepidodendraceae, sulfur butterfly, turn a blind eye, zannichelliaceae

17 letters:
australian magpie, cabbage butterfly, emperor butterfly, give the glad eye, monarch butterfly, peacock butterfly, sulphur butterfly

18 letters:
milkweed butterfly

20 letters:
steak and kidney pie

23 letters:
mediterranean fruit fly

24 letters:
mourning cloak butterfly

26 letters:
southern cabbage butterfly

10 letters:
dehumidify