I have a customer that wants a two tier cake to serve 21 people. What size cake pans should I do? I have an 2 8 inches baked already I just don't know if I should bake two 10 in or 6 in? Anyone?

If your using the wilton party chart, do a 6" and an 8". An 8 inch serves 20 and a 6" serves 12. You could also do one of those 4" cakes, I'm not sure how many they serve though.

http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm

A 4" serves 6.
A 7" serves 19.