The easiest way to know how much to eat is to take some time to learn the USDA Food Pyramid Guide. This takes only about a half an hour. Start by eating the lowest of the recommended number of servings in every food group, every day. As you'll see, the majority of serving sizes are given using a simple measuring cup as the basic frame of reference: one cup, one-half cup, one-quarter cup.
For a week or two, take a plastic one-cup measure around with you and just memorize sizes. One cup is 8 ounces of liquid. Learn by trial and error. For ounces, the size of a deck of regular playing cards is about the equivalent of 2-3 ounces. The size of your palm could indicate the right size meat (protein) serving.
The serving sizes are determined by a large group of nutrition professionals, trying to find an average size. After you make a small amount of effort to truly know what "normal" or "average" eating is, then it will become second nature to you.
Now, getting the power to instinctively follow what you learn and eat the right amount of food comes from God. Always ask for guidance and help, by praying to the divinity inside you.