If you type in 'D9P chart' in the search blank of this board, you will find a very handy size chart for making the D9P block. Sorry I don't know how to link to it.
But your finished D9P block will be 2" less than the sum of 3 cut squares. For example, if you cut the 9 squares 4" each, three of them = 12", and the finished block size will be 10". Your desired 9" finished size is a little odd, because each cut square will have to be 3 2/3". (3.6667 x 3= 11"). If you cut each square 3 3/4" you will have a finished D9P block just slightly larger than 9".