3 letters:
ism

5 letters:
agism, pisum, prism

6 letters:
ablism, autism, egoism, fatism, momism, nanism, obiism, papism, porism, racism, schism, sufism

7 letters:
agonism, animism, atheism, atomism, biprism, bossism, bromism, comtism, dadaism, dualism, egotism, elitism, erotism, fascism, fattism, fauvism, grecism, heroism, imagism, judaism, lamaism, leftism, locoism, onanism, saktism, sikhism, sophism, statism, toryism, tourism, tropism, zionism

8 letters:
acosmism, actinism, activism, albinism, algorism, alienism, alpinism, altruism, aneurism, aphorism, arianism, asterism, atticism, botulism, cabalism, cronyism, cynicism, dandyism, docetism, donatism, druidism, dynamism, embolism, ergotism, escapism, euphuism, exorcism, familism, fatalism, feminism, futurism, galenism, hebraism, hedonism, helotism, hinduism, humanism, iotacism, irishism, laconism, latinism, legalism, leninism, localism, logicism, lyricism, mazdaism, moralism, nativism, naturism, nepotism, nihilism, nomadism, optimism, pacifism, paganism, populism, ptyalism, puseyism, quietism, rightism, rigorism, romanism, rowdyism, royalism, ruralism, satanism, sciolism, semitism, shaktism, sinapism, solecism, stoicism, teratism, tokenism, totalism, totemism, ultraism, unionism, virilism, vitalism

9 letters:
amoralism, anatocism, anglicism, animalism, animatism, anomalism, atonalism, barbarism, behmenism, biblicism, biologism, briticism, brutalism, caesarism, calvinism, castroism, catechism, communism, cretinism, criticism, daltonism, darwinism, despotism, diabolism, dicrotism, dogmatism, epicurism, eroticism, erythrism, eunuchism, euphemism, exoticism, extremism, fabianism, fetichism, fetishism, feudalism, formalism, gallicism, galvanism, germanism, globalism, gothicism, grundyism, hellenism, herbalism, hirsutism, hitlerism, hoydenism, hybridism, hypnotism, isomerism, jansenism, jesuitism, lathyrism, magnetism, mannerism, mechanism, meliorism, mendelism, mentalism, mesmerism, meteorism, methodism, mithraism, modernism, monachism, mosaicism, muslimism, mutualism, mysticism, neologism, nephalism, ostracism, passivism, pauperism, pessimism, platonism, pluralism, quakerism, racialism, rhotacism, ritualism, saturnism, scientism, serialism, socialism, sovietism, spiritism, stalinism, syllogism, symbolism, synergism, terrorism, tribadism, verbalism, vishnuism, volcanism, voodooism, vulcanism, vulgarism, wahhabism, wellerism, witticism

10 letters:
absolutism, acephalism, adactylism, africanism, alcoholism, allomerism, amensalism, anabaptism, antagonism, archaicism, asceticism, automatism, autotelism, benthamism, bipedalism, boehmenism, bolshevism, brahmanism, brahminism, britishism, caffeinism, capitalism, catabolism, centralism, chauvinism, chromatism, cinchonism, classicism, dysphemism, ecotourism, empiricism, eudemonism, euhemerism, evangelism, fanaticism, favoritism, federalism, gargoylism, geotropism, gnosticism, gradualism, heathenism, hypericism, hypocorism, illuminism, initialism, insularism, invalidism, italianism, jacobinism, journalism, kantianism, katabolism, laconicism, lambdacism, liberalism, marcionism, metabolism, metamerism, militarism, minimalism, moderatism, monetarism, monogenism, morphinism, narcissism, naturalism, negativism, neutralism, nominalism, organicism, paralogism, patriotism, phallicism, pharisaism, plagiarism, polymerism, popularism, positivism, pragmatism, puritanism, pyrrhonism, quackerism, radicalism, recidivism, relativism, rheumatism, ruffianism, scepticism, schematism, scotticism, secularism, sensualism, separatism, skepticism, specialism, spoonerism, stigmatism, surrealism, syncretism, traumatism, trotskyism, utopianism, vaisnavism

11 letters:
academicism, achromatism, adoptionism, adventurism, agnosticism, agrarianism, allotropism, americanism, anachronism, anglicanism, anorchidism, arminianism, astigmatism, asynclitism, autoerotism, behaviorism, bohemianism, cannibalism, catholicism, clericalism, colonialism, communalism, consumerism, corporatism, creationism, criminalism, deaf-mutism, determinism, didacticism, eclecticism, ecumenicism, endaemonism, erastianism, ergotropism, esotericism, eudaemonism, externalism, favouritism, gallicanism, hegelianism, historicism, hooliganism, illusionism, imperialism, infantilism, libertinism, lutheranism, malapropism, materialism, medievalism, mennonitism, mithraicism, monasticism, nationalism, neuroticism, objectivism, officialism, opportunism, orientalism, originalism, panpsychism, parallelism, pastoralism, paternalism, pelagianism, personalism, physicalism, pleochroism, pointillism, primitivism, prochronism, proselytism, rationalism, reactionism, regionalism, religionism, revisionism, romanticism, socinianism, southernism, synchronism, syndicalism, systematism, tautomerism, teetotalism, vaishnavism, voluntarism, wesleyanism

12 letters:
abolitionism, aeroembolism, aestheticism, antifeminism, asynchronism, behaviourism, bilingualism, biomagnetism, bioterrorism, cartesianism, charlatanism, chemotropism, collectivism, commensalism, confucianism, conservatism, deviationism, diamagnetism, diastrophism, dichromatism, dilettantism, ecoterrorism, emotionalism, epicureanism, essentialism, evolutionism, expansionism, geomagnetism, heliotropism, hydrotropism, hypogonadism, intuitionism, isolationism, keynesianism, mendelianism, mercantilism, metachronism, metasomatism, mongolianism, monorchidism, necrophilism, neoplatonism, neurotropism, noctambulism, obscurantism, parachronism, parkinsonism, parochialism, philistinism, phototropism, polytonalism, reductionism, sabellianism, secessionism, sectarianism, sectionalism, somnambulism, subjectivism, syndactylism, traducianism, transvestism, universalism, voluntaryism

13 letters:
accidentalism, aldosteronism, anti-semitism, antinomianism, athanasianism, autoeroticism, bureaucratism, catastrophism, colloquialism, commercialism, conceptualism, ecumenicalism, equestrianism, exhibitionism, expressionism, functionalism, hermaphrodism, homoeroticism, homosexualism, immaterialism, impressionism, individualism, industrialism, malthusianism, meteortropism, microorganism, mohammadanism, mohammedanism, monometallism, monophysitism, muhammadanism, neo-darwinism, neoclassicism, neoliberalism, occasionalism, occidentalism, paramagnetism, particularism, perfectionism, phenomenalism, polydactylism, postmodernism, protectionism, protestantism, provincialism, republicanism, sacerdotalism, scholasticism, theanthropism, thermotropism, thigmotropism, trichromatism, trophotropism, unilateralism, vernacularism

14 letters:
able-bodiedism, abstractionism, antiquarianism, associationism, clinocephalism, constructivism, cryptorchidism, egalitarianism, evangelicalism, existentialism, ferrimagnetism, ferromagnetism, fundamentalism, holometabolism, hypoadrenalism, hypothyroidism, metachromatism, millenarianism, monochromatism, narcoterrorism, neocolonialism, neoromanticism, obstructionism, operationalism, paleomagnetism, pentecostalism, peripateticism, photoperiodism, prescriptivism, pythagoreanism, rastafarianism, rosicrucianism, sacramentalism, self-criticism, sensationalism, sentimentalism, servomechanism, traditionalism, transsexualism, transvestitism, ultramontanism, zoroastrianism

15 letters:
aristotelianism, confessionalism, conventionalism, cosmopolitanism, ecclesiasticism, episcopalianism, experimentalism, heterosexualism, hyperadrenalism, hyperinsulinism, hyperthyroidism, instrumentalism, intellectualism, interventionism, photojournalism, presbyterianism, professionalism, stereoisomerism, supernaturalism, thromboembolism, totalitarianism

16 letters:
anti-catholicism, authoritarianism, configurationism, consequentialism, counterterrorism, electromagnetism, environmentalism, epiphenomenalism, internationalism, machiavellianism, marxism-leninism, neoexpressionism, ultranationalism

17 letters:
anglo-catholicism, congregationalism, constitutionalism, deconstructionism, denominationalism, predestinarianism, transcendentalism, valetudinarianism

18 letters:
antiferromagnetism, hyperaldosteronism, hypoparathyroidism

19 letters:
hyperparathyroidism, hypoadrenocorticism

20 letters:
hyperadrenocorticism

9 letters:
mormonism