I am not trying to put you off by any means, but the size is really up to you. I would suggest making some samples of 5" blocks, 6.5" block or even the 10" block suggested above. You don't even have to sew the little corners in place. Just fold on the diagonal and lay on the square block to see what size corner pleases your eye.
Thanks for your question. It reminded me of this easy and fast block that I had forgotten about. Time to make a snowball quilt!