Sheet cakes comes in different sizes because of the many different pans available.

Serving size is a matter of personal preference, type of event, and budget.  For casual events, you may want to increase the serving size. For more formal affairs, you may want to reduce the serving size.

Commercial bakeries with commercial ovens often use the full sheet pan. This pan will not fit inside the standard home oven.

The rectangular pan that most home bakers are familiar with (one box of Pillsbury, Duncan Hines or Betty Crocker) is the quarter sheet pan, which measures approximately 9" x 13", with a depth of either 2 or 3 inches. If you lay 4 of these pans together, it would be the approximate size of a full sheet.

Sheet cakes from a bakery come in three basic sizes.

Notice that the 1/2 sheet cake is neither twice the size of a 1/4 cake or half the size of a full sheet cake. It's actually only 50% larger than a 1/4 cake.