3 syllables:
mystically

4 syllables:
artistically, autistically, ballistically, casuistically, cladistically, deistically, eristically, euphuistically, faunistically, heuristically, holistically, hubristically, juristically, linguistically, logistically, meristically, monistically, puristically, sadistically, simplistically, sophistically, statistically, stylistically, theistically

5 syllables:
agonistically, altruistically, aoristically, aphoristically, atavistically, atheistically, atomistically, cabalistically, calvinistically, chauvinistically, communistically, cross-linguistically, dioristically, dualistically, dyslogistically, egoistically, egotistically, eulogistically, euphemistically, fatalistically, formalistically, hedonistically, hellenistically, humanistically, imagistically, inartistically, jingoistically, journalistically, kabbalistically, legalistically, mechanistically, moralistically, narcissistically, nationalistically, naturalistically, nihilistically, novelistically, optimistically, pantheistically, pessimistically, pietistically, pluralistically, pugilistically, rationalistically, realistically, socialistically, solecistically, syllogistically, synchronistically, synergistically, urbanistically, voyeuristically

6 syllables:
anachronistically, anomalistically, antagonistically, cannibalistically, capitalistically, characteristically, collectivistically, deterministically, expressionistically, hypocoristically, idealistically, illusionistically, impressionistically, militaristically, monopolistically, monotheistically, mutualistically, opportunistically, paternalistically, polytheistically, positivistically, probabilistically, relativistically, ritualistically, somnambulistically, surrealistically, unrealistically

7 syllables:
exhibitionistically, imperialistically, materialistically, overoptimistically, phenomenalistically, sociolinguistically, uncharacteristically

8 syllables:
individualistically