I am torn between the size of a cake to serve 35-40
12' round - 2 layers or 3 layers?
10' or 12' square 2 layers?
Any advice out there?
The standard tier is 2 layers - - it's up to you if you want to make it 3 layers, but it won't affect the number of servings. A 12" round has 56 servings, while a 10" square has 50 and a 12" square has 72. These are Wilton standard wedding servings which are 1x2x4. If the servings are cut larger, there will be fewer of them.
I would determine how many servings are needed and go from there. HTH