You can make yours any size you like. Ideally, you need a clear acrylic template to help you frame and square them, but other than that, you can go for whatever size suits you.
Why 3 & 1/2??
Well, I got a set of acrylic templates when I started making these squares, and out of the different sizes that were in the pack, it just seemed like a manageable size!
Some of the squares and blocks I’ve done in this way have had totally scrappy centers—just pulled from my scrap bin at random and using the whites or blacks to frame them.