3 syllables:
mentalist

4 syllables:
the mentalist

5 syllables:
continentalist, fundamentalist, governmentalist, instrumentalist, orientalist, ornamentalist, sacramentalist

6 syllables:
environmentalist, experimentalist

3 syllables:
prevalent, relevant, renascent, specialist, eloquent, negligent, heaven-sent, estimate, deficit, decadent, prejudiced, pestilent, beguilement, settlement, detriment, evident, definite, pediment, enthrallment, testament, delicate, resonant, temperament, celebrant, delegate, petulant, excellent, feculent, sentiment, element, desolate, redolent, etiquette, sediment, elegant, emigrant, registrant, elephant, recusant, eminent, excrement, methodist, regiment, curettement, tegument, exquisite, pessimist, tenement, geminate, secondment, requisite, desiccant, vellicate

4 syllables:
bedevilment, inelegant, intelligent, irrelevant, integument, impediment, embezzlement, malevolent, prerequisite, affectionate, expressionist, geneticist, federalist, ingeminate, theosophist, maleficent, anesthetist, areca nut, new testament, nonresident, resettlement, perfectionist, development, executant, presentiment, old testament, preeminent, trace element, benevolent, white elephant, antecedent, connecticut, envelopment, sea elephant, trade deficit, indefinite, impressionist, ear specialist, heart specialist, nonresonant, predestinate, rogue elephant, self-evident, unprejudiced

5 syllables:
antifeminist, redevelopment, underestimate, budget deficit, picture element, frontier settlement, land development, logic element, marriage settlement, nondevelopment, rare-earth element, threshold element, unintelligent

6 syllables:
chemical element, underdevelopment, african elephant, ribbon development, housing development, federal deficit, fingerprint specialist, indian elephant, medical specialist, metallic element, property settlement, water development

7 syllables:
identity element, arrested development, viatical settlement, alluvial sediment, imperial elephant

8 syllables:
capital of connecticut

9 syllables:
housing and urban development

13 syllables:
department of housing and urban development

14 syllables:
secretary of housing and urban development

16 syllables:
international bank for reconstruction and development