Things to consider when choosing your cake sizes:

How many people you are serving.
How many tiers do you want your cake to be?
Are you saving the smallest tier for your first anniversary?
(If so, do not include it in the number of servings counted for the guests.)

Wedding cake serving size is 1"x 2"x cake height (4.5").

On average about 2/3 of all invited guest come. (Less if most of your guests live out of
town, and more if the majority live locally.)

Often an invited guest will bring a companion, or an invited couple may bring their
children. Order your cake size by the number of guests you expect and not the number
of invitations you sent.

If you are having a groom's cake, you can divide the number of servings between the two
cakes. The bridal cake is usually the larger of the two.

Extra Info:
A cake tier is 2 layers of cake iced together with a filling in the middle, making it 4.5" tall.
An average wedding cake is 3 or 4 tiers.