I'm baking a 2 tiered rectangular cake for 30people, what size mold should I use ?

thanks

You mean pan? I'd bake two 7x11 pans to make two layers and that should be enough for 30.

http://www.wilton.com/cakes/making-cakes/baking-serving-guide.cfm

bake a 9" square cake at least 4" tall --  then cut 3" off that so you have two pieces a 3" by 9" cake and a 6" by 9" cake then cut another three inches off the smaller one to make a 3" x 6 " cake to stack onto your existing 6" by 9" --

it's much easier to do than write -- that's over 30 servings

oh did you mean tiers or layers?

I meant layers and pans...English isn't my first language! 

Thanks for all your responses!