I have plans to make one of these. I was going to cut the squares to be 6 inches, and the shadows to be 1.5, so the finished block would be 7.5, and then use maybe a 2.5" border between blocks.
I was also planning to cut a strip of black that was 5.5" x wof and a strip of white that was 2" by wof, sew together, and then cut my shadows from there. I'd repeat with a 6" strip of black and another strip of white for the longer side of the shadow.
Hope that makes sense -- that's off the top of my head and not from my sketch book!