I like the corners of the snowball to cover the corners but not meet in the middle. If you use a 5 inch corner square, it will look more like a diamond in a square.
I like to base it on a 9-patch measurement so that 1/3 is a corner, 1/3 is the center portion, and 1/3 the other corner. The center then becomes a hexagon (6 sides). For a 10" block I would cut 3-5/8 (including seam allowances).