You can use any size squares. Charms squares 5" would work well. I used 5-1/2" because I was using squares from a panel for mine. If you make the center square all the same fabric (or color) it makes a nice pattern. Although I've seen some random ones that are very pretty too. I also kept the darker ones in the middle of each side, the panel squares on 2 corners and lighter ones on the other 2 corners. I'm glad I did a test block, because I had the lights/darks reversed and didn't like it as much that way.
I'll attach a picture of mine so you can see what I mean about the centers. Mine are all green.