36x36+ is a good size to lay on the floor for a baby to hang out on, it is also a good size to throw over a car seat while going from the house to the car during cold or wet weather. I crib matress is 27x52.
I just did one that is 39x57 and that sucker is HUGE. I used a "baby quilt" pattern to make it and I think it is unreasonably large!