What's the biggest square you can make with a 70" diameter circle- so it doesn't have curved sides?

i want to make curtains with some fabric but i can't find any M&M fabric in print right now. So i'm looking to buy the M&M'S 70" Round Fleece Blanket, 70" diameter, and I want to know how many i might need to buy. What's the largest rectangle/square that I can get without having curved sides out of a 70" diameter circle?