The Collaborative International Dictionary of English v.0.48:

Sizer \Siz"er\, n.
   1. See Sizar.
      [1913 Webster]

   2. (Mech.)
      (a) An instrument or contrivance to size articles, or to
          determine their size by a standard, or to separate and
          distribute them according to size.
      (b) An instrument or tool for bringing anything to an
          exact size.
          [1913 Webster]