my sizes are: large enough to cover the kid and not too big for the bed......
However, most of my charity quilts are about 36"x45" I find that is the best use of the fabric.
Rarely do I make one the width of the fabric 45"x54"
If you are using a packaged batting, then use that size. Warm and Natural comes in a craft size of 34"x43" or thereabout.