I have seen them from 36" square to my usual size which is 40x54. I plan on a quilt to be used by a child for several years, first in a crib, then as a drag around and TV quilt. The 40" is because I don't want to piece the back. If I have wider backing, I go with 45x54, the size of Thermore crib batting. I have seen my quilts used in the bottom of a playpen. That works too. Sandilee is right that the size is up to you.