By the way, you are now listing different values without your post now (15x13 becomes 13x11)
Admittedly, I too thought it was better not to count the arena's outer walls, but either way the value should stay consistent.
Also, sometimes I can't view the GB image (though it can be done by copying and pasting it in the url bar) so you may want to find another (though I'm sure we all know what GB arenas were like)