Hi purple,
this is the way I have it figured.
You are going to make a pinwheel block that is basically a four patch, each one made of half square triangles right..
So, if the finished size of the whole block is 6 inches then your four patches will be 3 inch finished.
To cut the half square triangles for the 3 inch finished patch, cut your squares 3 7/8 and cut in half diagonally. when you sew it back together, you will have a 3 1/2 square that will finish 3 inches.
try it with a scrap of something else and make sure it comes out right before you cut the fabrics that you are short on.