5 letters:
italy

6 letters:
infamy, wicopy, dimity, wiggly, fixity, litany, bigamy, sicily, realty, simile

7 letters:
visibly, timpani, tiffany, timothy, willowy, dittany, acidity, aridity, fidgety, civilly, vividly, trilogy, finicky, trinity, imagery, bigotry, mimicry, avidity, dingily, mistily, rickety, dignity, tripoli, rigidly, timidly, infancy, syncope, giddily, dizzily, epitome, hymnody, licitly, millime, ribbony, twinkly, windily, wriggly, fishily

8 letters:
polygyny, symphony, vicinity, visually, tinselly, salinity, exiguity, filthily, sympathy, stickily, solidity, signally, rigidity, ministry, rapidity, tumidity, vapidity, cupidity, toxicity, fluidity, humidity, timidity, filigree, symmetry, affinity, ubiquity, stingily, motivity, sapidity, brittany, infinity, instancy, villainy, frigidly, dismally, wickedly, epiphany, industry, fiscally, wilfully, polygamy, tepidity, limpidly, lividity, impishly, felicity, antigone, activity, tonicity, validity, lucidity, finitely, iniquity, minutely, viridity, acridity, piquancy, divinity, chivalry, infantry, nativity, exigency, bishopry, flindosy, gelidity, prissily, trickily, viscidly, gibingly, pidlimdi

9 letters:
gimmickry, viscidity, facsimile, captivity, synchrony, torridity, indignity, antiquity, anonymity, typically, publicity, invisibly, flippancy, viciously, ethnicity, liquidity, fittingly, acclivity, atonicity, stupidity, instantly, turbidity, putridity, antipathy, benignity, cyclicity, initially, polyphony, hillbilly, torpidity, declivity, soliloquy, duplicity, abysmally, miserably, cynically, proximity, obliquity, limpidity, turgidity, willingly, festivity, placidity, literally, willfully, aborigine, immorally, liberally, sublimity, unanimity, virginity, frigidity, prolixity, culicidae, killingly, wittingly, asymmetry, malignity, morbidity, stolidity, passivity, gravidity, irritably, chintzily, lingually, rusticity, skilfully, winsomely, assiduity, uncivilly

10 letters:
proclivity, sanguinity, skillfully, magistracy, benignancy, critically, excitingly, gravimetry, admittedly, malignancy, plasticity, relativity, stringency, creativity, implicitly, alkalinity, resiliency, originally, brilliancy, swimmingly, elasticity, derisively, resignedly, spitefully, deficiency, spasticity, officially, complicity, asynchrony, negativity, trippingly, physically, invalidity, infelicity, simplicity, flaccidity, pasigraphy, flippantly, insipidity, femininity, explicitly, incisively, positivity, inactivity, sphericity, somniloquy, equanimity, efficiency, reactivity, forgivably, insistency, mendicancy, minstrelsy, pipal tree, cliquishly, synonymity

11 letters:
exquisitely, proficiency, expansivity, whimsically, alkalimetry, befittingly, receptivity, objectivity, whiffletree, contingency, masculinity, obstinately, astringency, pellucidity, periodicity, domesticity, antistrophe, ambitiously, brilliantly, delinquency, deliciously, principally, magnanimity, apocynaceae, ellipticity, politically, sensitivity, resistivity, religiously, catholicity, unwillingly, electricity, intrepidity, sufficiency, longanimity, directivity, calorimetry, judiciously, unwittingly, serendipity, whippletree, differently, amphictyony, specificity, swingletree, consistency, reflexivity, colorimetry, efficiently, impassivity, selectivity, calligraphy, analyticity, permissibly, persistency, persnickety, propinquity, quizzically, reluctivity, retentivity, stringently, syllabicity, unthinkably, ventriloquy, cryptically, effectivity, forgivingly, lingeringly, obsessivity, pacifically, sickeningly

12 letters:
sufficiently, terrifically, refractivity, submissively, idiosyncrasy, eccentricity, absorptivity, auspiciously, unofficially, individually, indecisively, subjectivity, perniciously, inefficiency, spheniscidae, artificially, conductivity, sweet cicely, holy trinity, deliberately, transitivity, peripherally, additionally, multiplicity, capriciously, perceptivity, vindictively, consistently, chromaticity, specifically, fortuitously, indistinctly, productivity, irresistibly, uncritically, propitiously, avariciously, convincingly, authenticity, inelasticity, inhospitably, oil industry, reflectivity, semifluidity, sex activity, shittah tree, toy industry, unforgivably, bewitchingly, overactivity, unthinkingly

13 letters:
intermittency, cucurbitaceae, expeditiously, insufficiency, conditionally, realistically, concentricity, unequivocally, hyperactivity, intrinsically, supersymmetry, injudiciously, synchronicity, radioactivity, pusillanimity, inconsistency, promiscuously, traditionally, inefficiently, unchristianly, insensitivity, ignominiously, statistically, arms industry, coal industry, consanguinity, correlativity, film industry, impermissibly, progressivity, shoe industry, sign industry, unambitiously, unpredictably, with sympathy, with validity, egotistically, interactivity, long-windedly, unforgivingly

14 letters:
inauspiciously, superciliously, insufficiently, optimistically, scientifically, bioelectricity, meretriciously, intransitivity, linguistically, unconvincingly, human activity, inconsistently, mental imagery, movie industry, sapodilla tree, steel industry, taoist trinity, unpropitiously, altruistically

15 letters:
radial symmetry, surreptitiously, sexual activity, unrealistically, neuropsychiatry, growth industry, unconditionally, blessed trinity, granadilla tree, mass deficiency, mirror symmetry, put differently, social activity, with efficiency, zinc deficiency, pessimistically, sidesplittingly

16 letters:
self-sufficiency, radiosensitivity, piezoelectricity, hydroelectricity, photosensitivity, lactobacillaceae, service industry, apparel industry, banking industry, capital of italy, chondrodystrophy, fashion industry, housing industry, radial asymmetry, sensory activity, tobacco industry, without sympathy

17 letters:
superconductivity, relative humidity, chemical industry, mental deficiency, photoconductivity, thermoelectricity, aluminum industry, business activity, computer industry, higgledy-piggledy, lighting industry, plastics industry, refining industry, trucking industry

18 letters:
general relativity, bilateral symmetry, anthropocentricity, muscular dystrophy, characteristically, static electricity, special relativity, doctor of divinity, master of divinity, munitions industry, offensive activity

19 letters:
automobile industry, commercial activity, current electricity, dynamic electricity, securities industry, subversive activity

20 letters:
theory of relativity, juvenile delinquency, australian aborigine, bachelor of divinity, educational activity, electronics industry, uncharacteristically

21 letters:
construction industry, intelligence activity, shipbuilding industry

22 letters:
entertainment industry

23 letters:
atmospheric electricity

24 letters:
extracurricular activity

25 letters:
coefficient of elasticity

27 letters:
conservation of electricity

28 letters:
general theory of relativity, special theory of relativity