How much a person eats should be determined on a individual basis. The trick is to learn what your body can tolerate. Eat too much and you might suffer nausea or intestinal distress. Eat too little and you'll run out of energy. Generally, you want your pre-exercise meal to be a smaller snack.
Intensity is also a factor to consider.
An extremely heavy workout slows digestion so that more blood can be sent to your muscles. So if you're planning to work hard, have a smaller snack or eat further ahead of schedule. Avoid fatty foods. Fried foods, doughnuts, pizza, potato chips, etc., all contain lots of fat. Fat leaves your stomach slowly, and may cause nausea and a general bloated feeling during exercise.