Hopefully someone will back me up, but, I think the square for the four corners should be at least 3 7/8". While the finished size is 3", the triangle is actually being sewn onto a 3 1/2" square. Also, though I am not positive, I think the setting square that will be cut in 4 should be more like 6". 5 1/2 might just make it, but you lose some with the triangle points when sewing them on. Mudd?