The Collaborative International Dictionary of English v.0.48:

Proportionable \Pro*por"tion*a*ble\, a.
   Capable of being proportioned, or made proportional; also,
   proportional; proportionate. -- Pro*por"tion*a*ble*ness, n.
   [1913 Webster]

         But eloquence may exist without a proportionable degree
         of wisdom.                               --Burke.
   [1913 Webster]

         Proportionable, which is no longer much favored, was of
         our [i. e., English writers'] own coining. --Fitzed.
                                                  Hall.
   [1913 Webster]

WordNet (r) 3.0 (2006):

proportionable
    adj 1: proportionate