Whatever size you want, as long as they are all the same size so that when you attach the next block, the secondary pattern (little squares on point) are formed. Unless you want a wonky look - also a possibility.
So if you start with a 9" finished block, you can use anything from 1" squares to 3" squares or even larger (remember to add the 1/2" seam allowance). It depends on the look you want.