The best jean pocket size is one that is in proportion to your butt.
Obviously, a larger backside means a larger denim pocket, but if you get too large, you take the risk of looking masculine. Too small and your backside appears to be larger.
But, don't go getting out the tape measure and making the size of the pocket your first and only rule to follow.
If you were to compare two denim pockets that are the same size, a pocket that has more embellishments on it will appear to be smaller than a plain pocket. So, for this rule of thumb, your keen sense of style should be your indicator of the correct size needed for the best fitting jeans.