3 syllables:
atomists

6 syllables:
neuroanatomists

3 syllables:
weightlessness, faithlessness, gracelessness, balefulness, playfulness, graciousness, patroness, shamefulness, ailanthus, brazenness, maintenance, stableness, agelessness, spaciousness, gracefulness, agedness, painfulness, wakefulness, blamelessness, aimlessness, wastefulness, abacus, abstinence, accidence, accidents, acinous, activists, adamance, advocates, agamous, ambulance, ampleness, anabrus, analysts, angelus, anginous, annulus, antrozous, anulus, anxiousness, appetence, assonance, averageness, avidness, balanus, bashfulness, blasphemous, cabassous, cabinets, calamus, calculus, callousness, camelus, cancellous, candidness, cannabis, crabbedness, cracticus, crapulence, crapulous, fabulous, famulus, fathomless, fatuous, flatulence, fractiousness, fraxinus, gladfulness, gladsomeness, graduates, granicus, handedness, handleless, handsomeness, jaculus, jaggedness, manfulness, nabalus, naturalness, pallidness, raggedness, randomness, ravenous, sacculus, scandalous, tactfulness, tactlessness, tantalus, thalamus, thankfulness, tragulus, zalophus

4 syllables:
loquaciousness, salaciousness, rapaciousness, audaciousness, fugaciousness, voraciousness, capaciousness, unfaithfulness, capableness, acceptableness, ambassadress, anachronous, analogous, analysis, anatomise, antagonise, calamitous, capacitance, dialysis, elaborateness, extravagance, gelatinous, inhabitants, intransigence, invalidness, left-handedness, magnanimous, metabolous, miraculous, misshapenness, paralysis, recalcitrance, right-handedness, substantialness, the calculus, unanimous, unnaturalness

5 syllables:
mucilaginous, availableness, charitableness, ametabolous, cost analysis, genus cannabis, leptodactylus, macrodactylus, oleaginous, openhandedness, pterodactylus, pusillanimous, renal calculus, self-analysis, uranalysis

6 syllables:
inefficaciousness, artiodactylous, cystoparalysis, flaccid paralysis, functional calculus, haemodialysis, hemimetabolous, hemodialysis, holometabolous, integral calculus, predicate calculus, psychoanalysis, spectrum analysis, systems analysis

7 syllables:
chemical analysis, critical analysis, differential calculus, eleutherodactylus, harmonic analysis, heterometabolous, infantile paralysis, in the last analysis, regression analysis, urinary calculus

8 syllables:
gravimetric analysis, in the final analysis, qualitative analysis, quantitative analysis

9 syllables:
infinitesimal calculus

10 syllables:
methodological analysis

11 syllables:
quantitative chemical analysis