A 9 x 13 inch, single-layer cake yields 15 to 20 servings. A double-layer, 9 x 13 inch cake that is at least 4 inches tall yields 36 party-size servings or 50 wedding-size servings, which traditionally are slightly smaller.
According to Wilton, party-size cake pieces are cut to 1 1/2 x 2 inches. Wedding-size pieces are cut to 1 x 2 inches. The 9 x 13 inch cake is also known as a quarter-sheet cake. The quarter-sheet size is recommended for small birthday parties, office gatherings or intimate weddings. A half-sheet cake yields up to 54 party-size pieces and 74 wedding-size portions.