If in doubt, call ahead and make sure that your special needs can be accommodated. Ask if you can order a petite size, or if they allow you to share an entree. Ask if you can order a la carte. Never go to a restaurant feeling ravenous. Don't make the mistake of thinking that since you are paying for this meal that means you have to eat everything in sight.
Buffets can be deadly.
Beware of the clean plate syndrome. There are other ways of helping starving children. Don't let yourself be swayed by peer or family pressure, decide what you want and order it. Be aware that beverages, alcoholic or not, can contain a lot of calories. Ask for salad dressings, sauces and toppings on the side.