Gay has the right numbers and the good strategy for figuring it out via drawing. Mathematically , the exact size to make the square is 7.2071 inches, but for real world utility, it's best to round that to 7.25 inches and then trim them down a bit after they are sewn. (If you you sew an exact .25 inch seam, the triangle will be a tiny bit less than 2 mm larger than the intended 6 inches.) Or maybe just let them be the tiny bit larger than 6 inches.