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" Double Layer
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
 
The tier size chart below 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. 
As a rule, if stacking directly on top of one another,
there needs to be 3-4 inches between tier sizes.
 
Double Layer Stacked Round Set-Up Servings (not including top layer)
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
 
 
Square & Rectangle 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
 Quarter sheet 9x13 20 (2x3), 30 (2x2)
Mid-size sheet 11x15 27 (2x3), 35 (2x2)
1/2 sheet 12x18 36(2x3),  54 (2x2),  84 (1x2 - wedding)

Full Sheet 18x24

Double 1/2 sheet
 
Heart Shape Servings
6 inch 8
9 inch 18
12 inch 40
15 inch 65
   
Hexagon Shape Servings
6 inch 9
9 inch 18
12 inch 45
15 inch 65