You can ultimately be as large as you want within reason (probably can't exceed 11" x 8") depending on how much time and effort you're willing to invest in Penis Enlargement. However keep in mind that once you hit 8" x 6" and above, you are undoubtedly in the top 1% of penis sizes, and for every inch you add on after that you're increasing the chances that intercourse will be more painful then pleasurable.
I myself am shooting for 8.5" x 6.25"...I'm not even totally sure if I'll wind up wanting to be that big, but I'll guage my decision as I approach my goal. With a length of 10", you're not going to completely penetrate most girls, because you'll find yourself bottoming out at around 8.75" or so, so that's going to leave you with 2.25" that aren't being used at all...
In the end it's your decision, but you may want to reconsider your goals. I'm not trying to discourage you at all, I just want you to be happy with what you end up with and not wish that you'd stopped sooner.