Take a look at the chart here to see my recommended servings per cake size. Round and square servings are based on a 3 layer tier and approx. 1″ x 2″ slice. Sheet cake servings are based on 2 layers.
Keep in mind, at home we tend to cut the more generous party style slices (approx. 1.5″ x 2″). At events, the caterers will usually cut the more moderate event style slices (approx. 1″ x 2″). Are your guests more likely to just want a bite or two? Are they going to want seconds? Will there be other desserts available?