There's been several discussion on sizes on this site. I consider to be a 1/2 sheet cake. I usually tell people it serves 50, using 2x2 slices. Two Layer, Party Cake, Feeds How Many? - Cake Central.
But when you're hosting a larger event, a sheet cake provides the most efficient The dimensions of your cake, and the size of slice you cut, determine how many A slice 3 inches long by 1 1/2 inches wide should be large enough to satisfy.
Sheet Cake Feeds When we have family birthdays at our house I buy a 1/4 sheet cake. We usually have ten to twelve guests and it works fine.