I'd go with what Kwhite said...in between crib and twin. You could do a twin and be sure it's ok, but then again...twin is kinda too big to drag around if you want it to be used that way.
I have a 4 year old girl, and she's a wee bit taller than average but not by much. She's too big for a 45" square quilt. The width is ok, but no way to pull it up to her neck without her piddies showing. You could probably go 50" x 75 " and get a lot of use out of it!