I've made several 4-patch posies and I usually decide how big to cut the squares based on how many I can fit into the amount of fabric in the repeats, so as to waste the least amount possible. If it's a smaller print, then smaller size squares seem to work the best. I usually aim for 3" and then go up or down 1/4 to 1/2 inch till I arrive at a number that divides into the amount of fabric in the repeat with the least amount of waste. But don't go smaller than 2.5 or larger than 3.5"