The Collaborative International Dictionary of English v.0.48:

king-size \king-size\ king-sized \king-sized\adj.
   1. extra large; as, a king-size bed. [WordNet sense 1]
      [WordNet 1.5]

   Note: A king-size bed is the largest size generally sold
         commercially; it is larger than a queen-size, which
         in turn is larger than a full-size (or double) bed,
         and that in turn is larger than a twin-size (or
         single) bed. The mattress for a king-size bed is
         typically about 76 inches wide and 80 inches long.
         [PJC]

   2. Of or pertaining to a king-size bed; suitable for a
      king-size bed; as, a king-size bedcover.
      [PJC] Kingston

WordNet (r) 3.0 (2006):

king-sized
    adj 1: extra large; "a king-size bed" [syn: king-size, king-
           sized]