11x15 single layer cake.

35 based on 2x2x2" servings.

20 - 35 servings depending on how they're cut.

35 is for 2X2" servings, which is pretty small for my family!
20 is for honkin' big 2X4" pieces

2x2 servings would only be about 35 regardless of hight.
11 divided by 2 is 5.5
15 divided by 2 is 7.5

5.5 x 7.5 is 41.25 but the safer math is 5 x 7 giving you 35 servings

if you go to my website and go to the price list i have all cake servings based on how big you cut the cake (3 choices for wedding slice that are 4 inches tall and 3 choices for party cakes as well)

that way if you cut them 2x2, 2x3, or 3x3 you know how many servings you will get. I even made little styrofoam "cake slices" so people could see how big or tall a piece would be...im a visual person myself.

Thanks everyone, It for a baby shower and it needs to feed 30, so do you think I should go with a 12x18 to make sure every one gets cake.

I would stick with the 11x15

Thanks, I think I'll go with the 11x15. Off I go to baking.