First off, no question is stupid! If you've never been on an eating plan before, there's lot of confusing stuff and the only way to learn is to ask questions. To figure out a serving size, you need to measure the entire amount and divide it up. I subscribe to Cooking Light magazine, which is a great resource for learning to cook healthfully, and they usually tell you how many cups per serving. For example, they'll say "recipe serves 6, approximately one and one-half cups per serving". To help fill yourself up at dinner, it might help to start with a low point salad or soup, eat that first and then have your main meal. A lot of being satisifed with smaller portions has to do with how slowly you eat, you need to allow your stomach time to know it has food in it. Starting with a salad or soup can start the filling up process and you may find yourself full before you've finished the rest of your food -
Keep asking those questions!