Pan size quandry

I am a baker, and we often (most of the time) make rolls out of the focaccia to make individual buns. Works out great, and is much less "messy" than the traditional method. Just kind of eyeball (or scale them) to about 60-70 grams each, ball up, smash down slightly, and proof. Voila! Easy focaccia buns!

Either or both would work.
In addition to real estate (square inches) members have noted so far, you might want to consider how thick and soft or thin and crispy you want the focaccia to be. The 9x13 pan will give a thicker, softer bread, the 11x15 a thinner, perhaps more crispy one (depending on baking time).

It's easy to multiply out (assuming equal depth)- 10x15=150 sq in, 9x13=117, 11x15=165- clearly closer to 150. You can also do this with circles, by the way, without calculating the area . Square the diameter- a 10" circle will be 100 (not an area, just a number to compare), a 14' circle will be 196, about twice 100, so a 14' pizza is roughly 2- 10" pizzas, as an example.

I would use the 11 x 15 pan, and not spread the dough out to the corners.