Source : Webster's Revised Unabridged Dictionary (1913)

Proportionable \Pro*por"tion*a*ble\, a.
   Capable of being proportioned, or made proportional; also,
   proportional; proportionate. -- {Pro*por"tion*a*ble*ness}, n.

         But eloquence may exist without a proportionable degree
         of wisdom.                               --Burke.

         Proportionable, which is no longer much favored, was of
         our [i. e., English writers'] own coining. --Fitzed.
                                                  Hall.

Source : WordNet®

proportionable
     adj : proportionate