The following information will help you select the proper size and configuration of your cake.  The first chart shows the approximate numbers of servings that can be expected from the corresponding size of a 2-layer cake.
Round Shape	Servings: 4" x 2" x 1 3/8"
6 inch	10
8 inch	18
9 inch	23
10 inch	29
12 inch	41
14 inch	56
16 inch	73
18 inch	93
20 inch	114
22 inch	138
24 inch	165
sheet 9x13	20
sheet 11x15	25
1/2 sheet 12x18	36(2x3),  54 (2x2),  84 (1x2)
The tier size chart can be used as a guide to select the best configuration based on the number of guests you are expecting.  Because it is often the custom for the bride and groom to keep the top tiers, these yields exclude the top tier.
Round Tier Sizes	Servings
8-6	18
10-6	29
10-8	29
12-8-6	59
12-9-6	64
14-10-6	85
14-10-8	85
14-10-8-6	103
16-12-8	114
16-12-8-6	132
16-12-9-6	137
18-12-10-6	163
18-14-10-6	178
As a rule, if stacking, their needs to be 4 inches between tier sizes.
Square Sizes	Servings
6 inch	12
8 inch	24
9 inch	32
10 inch	40
12 inch	60
14 inch	84
16 inch	112
18 inch	144
Heart Shape	Servings
6 inch	8
9 inch	18
12 inch	40
15 inch	65
Oval Shape	Servings
5 X 7 inch	10
7 X 10 inch	25
9 X 13 inch	40
12 X 16 inch	65
Petal Shape	Servings
6 inch	5
9 inch	15
12 inch	32
15 inch	55
Hexagon Shape	Servings
6 inch	9
9 inch	18
12 inch	45
15 inch	65