3 syllables:
synthesis, synthesise

4 syllables:
resynthesis

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

6 syllables:
amorphosynthesis, electrosynthesis, nucleosynthesis, osteosynthesis

2 syllables:
vileness

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

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

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

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

7 syllables:
ouranopithecus, reinforcing stimulus

8 syllables:
genus australopithecus

10 syllables:
negative reinforcing stimulus, positive reinforcing stimulus