3 syllables:
fretfulness

4 syllables:
forgetfulness

5 syllables:
self-forgetfulness

3 syllables:
emphasis, nebulous, elements, hephaestus, tetanus, sedulous, petulance, offenseless, fecklessness, membranous, restlessness, excellence, exodus, evidence, pendulous, recklessness, edifice, detritus, blessedness, helplessness, renascence, precedence, helpfulness, pestilence, tremulous, prevalence, eminence, tenebrous, aesculus, pegasus, endlessness, resinous, credulous, resonance, dreadfulness, elephants, eloquence, pleasantness, regulus, daedalus, gentleness, healthfulness, precipice, petalous, specialness, negligence, tendinous, tenuous, emulous, femaleness, genesis, xenopus, venomous, benelux, elegance, relevance, delegates, desmanthus, emphasise, estimates, friendlessness, gempylus, haemanthus, jejuneness, mespilus, methodists, penstemon deustus, precedents, preciousness, restfulness, senselessness, sentiments, settlements, sexlessness, specialists, temperateness, vengefulness, zestfulness

4 syllables:
contentiousness, benevolence, inelegance, antecedence, preeminence, beneficence, unpleasantness, affectedness, apetalous, directionless, edematous, shigellosis, state's evidence, maleficence, pink elephants, persepolis, incredulous, irrelevance, intelligence, malevolence, eleven-plus, necessitous, acceptableness, adenosis, agenesis, cnicus, defencelessness, defenselessness, developments, digenesis, distressfulness, dysgenesis, essentialness, good-temperedness, impetuousness, incessantness, licentiousness, momentousness, neglectfulness, oogenesis, pretentiousness, re-emphasise, relentlessness, respectfulness, successfulness, tendentiousness, unhealthfulness, unhelpfulness

5 syllables:
macrocephalous, leptocephalus, atelectasis, conscientiousness, hearsay evidence, hydrocephalus, direct evidence, microcephalous, overemphasis, acrocephalus, ambidextrousness, androgenesis, anencephalous, arctocephalus, autogenesis, biogenesis, blastogenesis, cacogenesis, caenogenesis, cainogenesis, cenogenesis, chirocephalus, cogent evidence, cynocephalus, cytogenesis, epigenesis, epinephelus, gynogenesis, kainogenesis, kenogenesis, metagenesis, microcephalus, monogenesis, moral excellence, myxocephalus, ontogenesis, palingenesis, pathogenesis, phylogenesis, pyrocephalus, rule of evidence, sacrilegiousness, sarcocephalus, stenopelmatus, unacceptableness, unpretentiousness, xenogenesis

6 syllables:
magnetic resonance, indirect evidence, dicotyledonous, abiogenesis, adult intelligence, agamogenesis, amelogenesis, anthropogenesis, atherogenesis, combat intelligence, current intelligence, funeral-residence, gorgonocephalus, heterocephalus, heterogenesis, nuclear resonance, parthenogenesis, platypoecilus, showing intelligence, signals intelligence, spermatogenesis, terrain intelligence, unconscientiousness, verbal intelligence

7 syllables:
circumstantial evidence, borderline intelligence, nonverbal intelligence, strategic intelligence, tactical intelligence

8 syllables:
artificial intelligence, electronics intelligence, proton magnetic resonance, security intelligence, showing lack of intelligence, telemetry intelligence

9 syllables:
communications intelligence