A crib mattress is 27 X 52". If you make it much bigger than that, it's hard to tuck into the crib. If you allow 6" on each side and at the foot, that would be a quilt of 39" X 58". But I don't want it to come all the way to the top (head) of the mattress, and we certainly don't need a pillow tuck, so I think that 39" X 52" is actually a good size if you want the quilt to be used in the crib.