As a general rule of thumb, if you have meat as the main dish, you need approximately 6 ounces per person, according to About.com. Vegetable side dishes should have 4 ounces per person. If you must estimate on portion sizes, round up to be on the safe side.
If you are serving rice or another grain as a side dish, you need 1.5 ounces per person. If the rice is more of a main dish, such as risotto, allow for 2 ounces each.
You need 5 ounces of potatoes, 2 ounces of beans and 2 ounces of pasta per person if those dishes are served as side dishes. When serving a salad, plan to have 1 ounce of lettuce per person. For desserts, plan to serve one slice of cake or pie to each guest, 5 ounces of ice cream or 4 ounces of a creamy dessert, such as a mousse.
Hors d'oeuvres call for four to six bites per person. If you are having a cocktail party, where the hors d'oeuvres are the entire meal, plan to have four to six bites of four or five different appetizers per person.
Be sure to estimate how long your dinner is likely to last. A longer dinner party with a larger guest list is likely to mean offering a larger variety of small bite dishes. About.com also recommends predicting the most popular foods at the dinner and planning to serve larger portions of those items.