This chart will give an approximate number of servings you can expect from a particular size cake.  It is my best estimate of the maximum number of servings you can expect.  Please note...The number of servings may vary when you cut your cake based on how big you cut the pieces, and also whether you have dowels in the cake.  If there are dowel supports in the cake, it may interfere with the cutting of some pieces, thereby reducing the number of servings you may actually get.  If you're in doubt about how much you need, you may want to order extra servings. 
Cake pieces are labeled either "party size" or "wedding size".  Traditionally, wedding sizes are a little smaller.  
Party size pieces on a 1-layer sheet cake are approximately 2" by 2" (by about 2" tall.)  
Party size pieces on a 2-layer cake are approximately 1.5" by 2" (by about 4" tall.)
Wedding size pieces on a 2-layer cake are approximately 1" by 2" (by about 4" tall.)  This size is pretty standard throughout the wedding cake industry.
Number of Servings in a Cake
Size:
Sheet
13x9
11x15
12x18
22x15
24x18

Round
4"
6"
8"
10"
12"
14"
16"
18"

Heart
6"
8"
10"
12"
14"

Square
4"
6"
8"
10"
12"
14"
16"

Texas
Shape
Small
Medium
Large

Custom
Cut-Outs
Variable
Party Size
1-Layer: 
24
35
54
77
100


2
4
8
18
28
38
44



5
11
14
20
28


4
9
16
25
36
49
64 



~8
~35
~48




Party Size
2-Layer: 
42
58
72
100
144


4
10
18
26
38
58
72



8
18
24
34
48


4
12
20
30
48
63
80 



~16
~50
~72




Wedding
2-Layer: 
50
75
105
150
210


4
12
24
38
56
78
100
127


14
22
38
56
72


8
18
32
50
72
98
128 



~20
~75
~105




Weddings...How many servings do I need?

There is no magic way of determining the exact number of servings you will need.  Too many factors will affect your attendance and the appetite of your guests.  Here are some things for you to consider...

1.  Think of your number of invitations.  Some have suggested that 1/2 - 3/4 of that number will attend.

2.  Some of the "invitations" that come to your wedding may bring along a date or a family of 4.

3.  Are you serving a sit-down dinner, finger foods, or only cake to eat?  These options will affect how "hungry" your guests are.

4.  Are you going to save the top tier for your first anniversary?  If so, do not count that when counting up "servable" cake for guests.  

5.  Are you having a groom's cake?  If so, that may reduce your number of needed bride cake servings.  Some folks enjoy a piece of BOTH cakes, while others will only choose one or the other.  Some say a good rule of thumb is to have 1/2 -3/4 the number of groom cake servings as you have bride cake servings.  

Again, these are just ideas to consider.  
Approximate number of servings:
Size of cake:
The "medium" and "large" Texas cakes are 2-D, custom-cut-out cakes.