The size of a cake that can serve 12 to 15 people depends on the serving size you prefer. However, as a general guideline, a 9-inch (22 cm) round cake or a 9x13-inch (23x33 cm) rectangular cake is typically recommended to serve around 12 to 15 people. This assumes that each person will be served a regular-sized slice of cake. If you prefer larger or smaller slices, you can adjust the size of the cake accordingly.