OP - that is only true of single layer (that is 2" high) sheet cakes, the width and length of the slices must be larger to compinsate for the height of the pieces being half that of layer cakes. I personally don't do single layer sheet cakes, so my serving size for rectangular cakes is 2"x1"x4" just like for any other cake.
The 2"x1"x4" serving size yields 8 cubic inches of cake. With a 2" high cake, to get the same volume of cake, the pieces must be cut larger (2"x2"x2").
A 4" high (2 layer) 9"x13" rectangular cake ("quarter sheet") will yield about 50 servings. A 2" high (single layer) 9"x13" rectangular cake ("quarter sheet) will yield about 25.
For any rectangular cake, to determine the number of servings, simply divide the surface area of the cake by the surface area of a piece of cake. Generally it's best to round down a bit from that number just to be on the safe side (most people don't cut pieces exact and its pretty much impossible to cut odd into even evenly).
HTH!
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