The two "Ornithomimus" species today seen as possibly valid differ in size. In 2010 Gregory S. Paul estimated the length of "O. edmontonicus" at , its weight at . One of its specimens, CMN 12228, preserves a femur (thigh bone) long. "O. velox", the type species of "Ornithomimus", is based on material of a smaller animal. Whereas the holotype of "O. edmontonicus", CMN 8632, preserves a second metacarpal eighty-four millimetres long, the same element with "O. velox" measures only fifty-three millimetres.