How big do you want it to be? The top of a full mattress is 54 X75, so if you want the star to be just the top of the mattress the closest options would be:
3.5" squares, finish at 3, star would be 48"
3.75" squares, finish at 3.25" would be a 52" star
4" squares, finish at 3.5, would be a 56" star.