3 syllables:
myologists

4 syllables:
corologis, ecologists, necrologists, zoologists, apologists, astrologists, biologists, chronologists, cosmologists, cryptologists, cytologists, ethnologists, ethologists, geologists, graphologists, histologists, horologists, hydrologists, ichthyologists, limnologists, mixologists, monologists, morphologists, mycologists, mythologists, nephrologists, neurologists, oncologists, ontologists, otologists, pathologists, pedologists, penologists, philologists, phrenologists, phycologists, proctologists, psephologists, psychologists, scatologists, seismologists, serologists, sexologists, sinologists, tautologists, technologists, topologists, urologists, virologists

5 syllables:
climatologists, ophthalmologists, volcanologists, acarologists, anthropologists, archaeologists, archeologists, audiologists, campanologists, cardiologists, cosmetologists, craniologists, creatologists, criminologists, demonologists, deontologists, dermatologists, embryologists, entomologists, escapologists, eschatologists, etymologists, futurologists, genealogists, gerontologists, glaciologists, gynaecologists, gynecologists, haematologists, hematologists, herpetologists, ideologists, immunologists, kremlinologists, laryngologists, malacologists, mineralogists, musicologists, numerologists, odontologists, oenologists, ornithologists, osteologists, pharmacologists, physiologists, radiologists, reflexologists, rheumatologists, scientologists, semiologists, sociologists, speleologists, teratologists, toxicologists, vexillologists, vulcanologists

6 syllables:
microbiologists, bacteriologists, biotechnologists, cryobiologists, cryptozoologists, endocrinologist's, endocrinologists, epistemologists, kinesiologists, meteorologist's, meteorologists, neonatologists, neurobiologists, neuropsychologists, paleontologists, parapsychologists, parasitologists, phenomenologists, psychobiologists, psychopathologists, sovietologists, venereologists

7 syllables:
anaesthesiologists, epidemiologists, ethnomusicologists, gastroenterologists, otolaryngologists, psychopharmacologists, radiobiologists

9 syllables:
otorhinolaryngologists

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

4 syllables:
predominance, self-confidence, hypostasis, hydrolysis, hypothesis, endogamous, abdominous, metropolis, sarcophagus, synonymous, incontinence, incompetence, concomitance, monogynous, ignobleness, saprophagous, self-consciousness, recognizance, obsoleteness, monoclinous, necropolis, anonymous, propositus, passionateness, diplodocus, inconsequence, esophagus, reconnaissance, monotonous, convolvulus, monogamous, homologous, oppositeness, autonomous, incompleteness, apophysis, acropolis, nitrogenous, anthophilous, subconsciousness, acrogenous, acromphalus, ammotragus, androgenous, anthophagous, apogamous, apophasis, astomatous, autogenous, autologous, autolysis, carpophagous, cryptogamous, cytolysis, eponymous, exogamous, gymnopilus, hippotragus, idolatress, improvidence, lose consciousness, melophagus, obnoxiousness, oesophagus, panonychus, phytophagous, sarcolemnous, taurotragus, uncommonness, uropsilus, with confidence, zoophagous

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

6 syllables:
adrianopolis, bacteriolysis, bacteriophagous, megalobatrachus

7 syllables:
displaying incompetence, genus hippopotamus

8 syllables:
urinary incontinence, alcoholics anonymous, electronic reconnaissance

15 syllables:
international society for krishna consciousness