I usually make my crib quilts about 37 X 45. It really depends on the pattern. But with this size you can use 1 cut of fabric for the backing since most fabric is about 42 inches wide.
And a 1 1/2 yard cut works for the length. By the time fabric is washed and cut straight and allowing some for quilting loss. Work out a block size and see how it works out as a finished quilt. Don't forget to fiqure in the extra seams in the D9P.