I just saw this online, but I'm not sure they gave the actual measurements for the square. Try sewing 5 of the circles together in a cross shape, then measure the square area in the center. I know that's the cheater way, but it takes the math out of it.
Found it....
http://www.straw.com/equilters/libra...gallbaros.html
draw a square inside your circle and you've got the measurement you need.