Below is a standard chart which you may use to determine how much cake you will need for your event.
These serving sizes are approximate and depend on how your cake is cut. Make sure you have a professional cutting your wedding cake. If you want bigger slices, you need to account for that and choose a larger size cake to accomodate the serving size you want.
Wedding cake servings are based on a 1" X 2" slice • Party cake servings are generally 1.5" x 2"