Most packaged foods have the recommended portion on them. Next time you reach for a box of cereal or crackers, look at the nutrition facts label. These are almost always based on a 2000 calorie diet, so if you don't need that many calories or if you need more, (
approximate your needs with this tool) you'll have to adjust accordingly.
On the label, "Serving size" is how much you should be eating. They can write it in several ways. If it's a family-sized bag of chips, for instance, they may write it as
about 12 chips or
1 oz. There might also be a "Servings per container" section. This lets you know how many servings are inside the entire package. So again, if we're talking about chips, the serving per container might be
about 10. That means your portion size should be no more than 1/10th of the bag.
When talking about meat, one portion is about the size of a deck of playing cards.
For most raw fruits and veggies, eating more than is needed is not really an issue. There are little calories in many of them and they offer many health benefits. More is usually better.
Drinks are almost always referred to in 8 oz/1 cup. portions. Your basic red Solo cup is 16 oz, as are most dinnerware tumblers. The old standard is that you need 8 glasses of water per day, so that's 4 Solo cups of water. The serving size for milk, juice and most other beverages is also about 8 oz.