Yikes! This is a new one for me. I've been asked to make a cookie cake for 40 and I don't have a clue as to how big to make one-ANY help would be appreciated!! My thought were to use 2 rectangle (15x10) cookie sheets & set next to each other for a large sheet (these will be "covered" with frosting) but will this be enough? Would I double the cookie recipe per pan or would that be too much dough?? How do I get myself into these things???

That, I think, would give you about 35 servings. Treat is just as if it were a cake; advising customer to cut into 2" sq.
I have no idea how to advise you regarding how much dough
I do know I could make 2 10" rounds OR 3 9" hearts from one cake mix. Yes I'm talking cookies from a cake mix And I baked them in my cake pans = didn't need any special pans.
Based on that info I would think you could use one mix per your pan. It should make a cookie about 1" thick. The cake mix cookies are delisious but they dry out quickly and need to be consumed within no more than 36 hrs after baking.

Thanks! I was going to try the cake mix cookie recipe here on CC, mmm...samples tonite for snacks!