6" round = 5 servings (only for tiered cakes)
8" round = 10 servings
10" round = 15 servings
12" round = 20 servings
14" round = 30 servings
Please note that most of my square and round cakes are Baked and filled to give you 4 layers of cakes and 3 layers of filling. These cakes will range between 3.5 and 4 inches tall. There are few exceptions when it comes to carved and sculpted cakes.
Please see the below chart for cake sizes and servings when trying to determine the right cake size for your event. A serving size is around 1" x 3" x 4" per slice. I use the industry standard Cake Sizes and Servings Chart. If you want bigger slices, you need to account for that and choose a larger size cake to accommodate the serving size you want.