Below is a standard chart which many bakers use to determine cake serving sizes.  
Wedding cake servings are based on a 1" X 2" X 4" slice, since other deserts will be
served.  Party cake servings are generally 1 1/2" x 2" x the height of the cake.  
These serving sizes are approximate and depend on how your cake is cut.
Reception halls charge an arm and your mother's leg to slice your cake for you, so
inquire as to how much they charge.   You can't afford to have Aunt Martha cutting
your wedding cake in pie wedges. Let her view the guide on how to cut the cake
appropriately.  Click here to link to Wilton's website which has a cake cutting guide.  
  Wedding Servings
2 layers
 Party Servings
2 layers
 Party Servings
1 layer - split
Oval
      
7 3/4" x 5 3/8"
 13
    
10 3/4" x 7 7/8"
 26
    
13 1/2" x 9 7/8"
 45
    
16 1/2" x 12 3/8"
 70
    
Round
      
5"
 10
 4
  
6"
 12
 6
  
7"
 19
 10
  
8"
 24
 15
  
9"
 32
 22
  
10"
 38
 30
  
12"
 56
 50
  
14"
 78
 70
  
16"
 100
 85
  
18"
 127
    
Petal
      
6"
 8
    
9"
 18
    
12"
 40
    
15"
 64
    
Hexagon
      
6"
 12
    
9"
 26
    
12"
 40
    
15"
 70
    
Heart
      
6"
 14
    
8"
 22
    
9"
 28
    
10"
 38
    
12"
 56
    
14"
 72
    
15"
 83
    
16"
 94
    
Square
      
6"
 18
    
8"
 32
    
9"
 42
    
10"
 50
    
12"
 72
    
14"
 98
    
16"
 128
    
18"
 162
    
Rectangle
      
9" x 13" (1/4sheet)
   48
 24
11" x 15" (1/3 sheet)
   70
 35
12" x 16" (1/2 sheet)
   100
 50
Popular Wedding Cake Tier Combinations

Round Cakes:            Square Cakes:
6"-8"-10" = 74 servings          6"-8"10" = 100 servings
6"-9"-12" = 100 servings          6"-9"-12" = 132 servings
6"-10"-14" = 128 servings          6"10"-14" = 166 servings
6"-8"-10"-12" = 130 servings          6"-8"-10"-12" = 172 servings
8"-10"-12" = 118 servings          8"-10"-12" = 154 servings
8"-12"-16" = 180 servings          8"-12"-16" = 232 servings
6"-10"-14"-18" = 255 servings          6"-10"-14"-18" = 328 servings
Cake Servings Chart