The Collaborative International Dictionary of English v.0.48:

Sizable \Siz"a*ble\, a.
   1. Of considerable size or bulk. "A sizable volume." --Bp.
      Hurd.
      [1913 Webster]

   2. Being of reasonable or suitable size; as, sizable timber;
      sizable bulk. --Arbuthnot.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sizable
    adj 1: fairly large; "a sizable fortune"; "an ample waistline";
           "of ample proportions" [syn: ample, sizable,
           sizeable]
    2: large in amount or extent or degree; "it cost a considerable
       amount"; "a goodly amount"; "received a hefty bonus"; "a
       respectable sum"; "a tidy sum of money"; "a sizable fortune"
       [syn: goodly, goodish, healthy, hefty, respectable,
       sizable, sizeable, tidy]

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "sizable":
   Gargantuan, amplitudinous, astronomical, awesome, big, boundless,
   bulky, bumper, colossal, considerable, cosmic, enormous, extensive,
   galactic, gigantic, good, goodly, grand, great, healthy, hefty,
   huge, immeasurable, immense, infinite, king-size, large,
   large-scale, major, mammoth, man-sized, massive, massy, monster,
   monstrous, monumental, mountainous, numerous, outsize, overgrown,
   prodigious, respectable, sensible, smart, spacious, stupendous,
   substantial, tall, tidy, titanic, tremendous, vast, voluminous,
   weighty