3 syllables:
onymous

4 syllables:
agronomists, allonymous, anonymous, anonymus, autonomous, economise, economist's, economists, eponymous, euonymous, heironimus, homonomous, hylonomus, paronymous, synonymous, theonomous

5 syllables:
heteronomous, heteronymous, nonautonomous, polyonomous

6 syllables:
semiautonomous

8 syllables:
alcoholics anonymous

3 syllables:
snobbishness, roguishness, boletus, complexness, bottomless, cognizance, columnists, commonness, communists, confidence, conscienceless, consciousness, consequence, consonance, continence, continents, documents, dominance, godlessness, honestness, modestness, monuments, noxiousness, obstinance, occupants, oculus, ominous, omnibus, omphalus, opulence, oxalis, pompousness, populace, populous, populus, profitless, prominence, prophetess, protestants, provenance, providence, romulus, rottenness, scolytus, slothfulness, somnolence, spotlessness, squalidness, stolidness, strophanthus, vomitus, watchfulness

4 syllables:
impoliteness, improvidence, responsiveness, inconspicuous, abdominous, acrogenous, acromphalus, acropolis, ammotragus, androgenous, anthophagous, apogamous, apophasis, astomatous, autogenous, autologous, autolysis, carpophagous, cryptogamous, cytolysis, endogamous, esophagus, exogamous, gymnopilus, hippotragus, homologous, hydrolysis, hypothesis, idolatress, incompetence, incontinence, lose consciousness, melophagus, metropolis, monogamous, monotonous, necropolis, nitrogenous, obnoxiousness, oesophagus, panonychus, pathologists, phytophagous, predominance, psychologists, recognizance, reconnaissance, saprophagous, sarcolemnous, sarcophagus, self-confidence, self-consciousness, subconsciousness, taurotragus, uncommonness, uropsilus, with confidence, zoophagous

5 syllables:
overconfidence, cumulonimbus, acantholysis, air reconnaissance, amylolysis, anisogamous, anthropophagus, campylotropous, creeping oxalis, disconsolateness, electrolysis, entomophilous, fibrinolysis, genus oxalis, haematolysis, hematolysis, heterologous, hippopotamus, loss of consciousness, megalopolis, nonnitrogenous, ophiophagus, proteolysis, semiconsciousness, stephanopoulos, stream of consciousness, unselfconsciousness, vote of confidence

6 syllables:
adrianopolis, bacteriolysis, bacteriophagous, megalobatrachus

7 syllables:
displaying incompetence, genus hippopotamus

8 syllables:
electronic reconnaissance, urinary incontinence

15 syllables:
international society for krishna consciousness