3 syllables:
sessional

4 syllables:
accessional, compressional, concessional, confessional, congressional, digressional, expressional, impressional, obsessional, possessional, precessional, processional, professional, progressional, recessional, secessional, successional, transgressional

5 syllables:
health professional, intercessional, non-professional, nonprofessional, preprofessional, subprofessional, unprofessional

6 syllables:
extraprofessional, paraprofessional, semi-professional, semiprofessional

3 syllables:
peaceable, regional, feasible, seasonal, teachable, beatable, reachable, squeezable, readable, eatable, aestival, bendable, chemical, credible, decimal, denticle, edental, edible, emmental, epical, epochal, equable, estival, ethical, festival, flexible, genital, getable, gettable, kettleful, legible, medical, meltable, methanal, peccable, pedestal, pennoncel, pentacle, pessimal, plentiful, preferable, rentable, requital, reticle, retinal, sceptical, seconal, sectional, sellable, seminal, seneschal, sensible, sensual, sentinel, skeletal, skeptical, spectacle, spendable, stretchable, tenable, tentacle, testable, vegetable, vegetal, vendable

4 syllables:
amenable, repeatable, unfeasible, appealable, redeemable, unspeakable, unreachable, infeasible, defeasible, appeasable, achievable, unreadable, believable, perceivable, reasonable, unbeatable, unreasonable, conceivable, alienable, acceptable, accessible, affectional, alchemical, amendable, ascendable, church festival, collectable, collectible, commendable, compensable, compressible, conceptional, congenital, contemptible, contestable, conventional, correctable, correctional, defendable, defensible, delectable, dependable, detectable, detestable, digestible, dimensional, directional, dispensable, electable, exceptional, execrable, expendable, expressible, extendable, extensible, forgettable, illegible, impeccable, impregnable, incredible, inedible, ineffable, inflectional, inflexible, injectable, insensible, intentional, intestinal, jazz festival, lamentable, ostensible, perceptible, polemical, preventable, raw vegetable, receptacle, redemptional, regrettable, respectable, resplendent quetzel, root vegetable, stand sentinel, susceptible, trigeminal, unbendable, unquenchable, unrentable, untellable, untenable

5 syllables:
inconceivable, unappeasable, indefeasible, unbelievable, unachievable, irredeemable, unappealable, irretrievable, unimpeachable, apprehensible, bidirectional, catechetical, commonsensible, comprehensible, hexadecimal, homosexual, hypothetical, inaccessible, incompressible, incontestable, incontestible, indefensible, indigestible, indispensable, inexpressible, inextensible, insurrectional, insusceptible, irrepressible, leafy vegetable, one-dimensional, reprehensible, third-dimensional, three-dimensional, two-dimensional, unacceptable, unaccessible, unamenable, unconventional, uncorrectable, undefendable, undependable, undetectable, unexceptional, unexpendable, unexpressible, unforgettable, uninjectable, unintentional, unpresentable, unpreventable, unrespectable, unsusceptible

6 syllables:
accounts receivable, heterosexual, multidimensional, omnidirectional, recurring decimal, religious festival, repeating decimal, unidimensional, unidirectional

7 syllables:
circulating decimal, electric receptacle