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Found one definition

1.                 From 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]