I am making a two layered birthday cake using the 3" deep Wilton round wedding cake pan set and filling the layers which if I remember right should give me a 4" deep cake. So I want to work on the fondant figures that will go on the sides but i am debating on how big or small I should make them.. I do not want to make them to small nor too big. What size should i stick to for the figures?

In advance thanks for your help

If you fill your pans to get 3" cakes, you will have over 6" of cake when it's filled and iced. If you fill them so you end up with cakes that are about 2", then you will end up with something slightly taller than 4".

The size of your figures will depend on the level of detail. If you don't want them to be taller than the cake, then simply make them shorter.

Yeah, what she ^^^^^ said
I wondered, as I read, how can two 3" deep layers yield a 4" tall cake
As for the figures, why not vary them some -- making some smaller than the other; still others even taller. Taht would give interest rather than seeing them all exactly the same size.