3 syllables:
synthesis, synthesise

4 syllables:
resynthesis

5 syllables:
biosynthesis, chemosynthesis, mental synthesis, narcosynthesis, parasynthesis, photosynthesis, photosynthesise, psychosynthesis

6 syllables:
amorphosynthesis, electrosynthesis, osteosynthesis

3 syllables:
tiddlywinks, wiliness, bibulous, blissfulness, brittleness, chivalrous, citrullus, dictamnus, diffidence, diligence, dissidents, dissonance, fickleness, fistulous, fitfulness, fixedness, frivolous, gibbousness, givenness, guiltlessness, immanence, immigrants, imminence, impetus, impotence, incidence, incidents, indolence, infamous, innocence, innocense, innocents, insolence, instruments, limitless, limulus, linanthus, lissomeness, listlessness, littleness, militance, militants, minuteness, mischievous, nicholas, nimbleness, nimravus, quiscalus, risklessness, shiftlessness, siliceous, simpleness, sinfulness, singleness, singultus, sinlessness, sinuousness, skillfulness, stimulus, syllabus, symptomless, synchronous, thriftlessness, tinnitus, vigilance, villainess, villainous, viscidness, viscousness, vividness, wickedness, wilfulness, willfulness, windlessness, winsomeness, wishfulness, wistfulness, wrinkleless

4 syllables:
unwillingness, vaginitis, acclivitous, acidulous, ambitionless, ambitiousness, ambivalence, antigonus, antithesis, assiduousness, asynchronous, auspiciousness, capriciousness, declivitous, deliciousness, equivalence, erithacus, explicitness, felicitous, grandiloquence, illicitness, implicitness, indigenous, insipidness, judiciousness, leviticus, magnificence, magniloquence, maliciousness, meticulous, munificence, nonsynchronous, nutritiousness, officiousness, omnipotence, participants, perniciousness, polygamous, propitiousness, religiousness, ridiculous, saint nicholas, suspiciousness, unchivalrous, unmindfulness, unskillfulness, utriculus, ventriculus

5 syllables:
cercopithecus, dryopithecus, expeditiousness, geosynchronous, inauspiciousness, inexplicitness, infelicitous, injudiciousness, irreligiousness, kenyapithecus, lufengpithecus, meretriciousness, nonequivalence, repetitiousness, rudapithecus, sivapithecus, unambiguous, unpropitiousness

6 syllables:
aegyptopithecus, algeripithecus, angle of incidence, australopithecus, aversive stimulus, conditioned stimulus, negative stimulus, positive stimulus, relative incidence

7 syllables:
ouranopithecus, reinforcing stimulus

8 syllables:
genus australopithecus

10 syllables:
negative reinforcing stimulus, positive reinforcing stimulus