A Thunder Power 900mah 11.1 is as large as you will want to go to keep the wing loading light! By going any larger you are killing performance in trade for duration.
I fly with the 480mah 11.1 for a super light wing loading but I also have about six of them charged to keep me flying.
So it really depends on the performance you want out of your plane! If you don't mind the extra weight go with the 900mah!
Hope this helps!