OKAY, I'm assuming you have the twister that cuts 3.5" squares. 3.5" squares finish at 3". The other thing to remember is that the tool goes on the seam, not the charm square, so you need 1 more square than the number of patches you want.
So say you want your small quilt to be 48 inches square. Divide 48 by 3 to get the number of starting squares you need, so 16 charm squares, plus one (because of the seam thing) so 17. so a starting charm square unit of 17 X17 charms will yield a 48" square twister.