We live in a super-sized food nation. Twenty years ago, a bagel weighed about two ounces and had less than 200 calories. Today, a typical bagel has doubled in size and calories.

And bagels aren't the only foods that have ballooned. Portion sizes in general are two to five times bigger than in the 1970s, according to New York University researcher Lisa Young. This slow but steady "portion distortion" is a major contributor to America's growing obesity epidemic. Even if you eat the right foods, you still have to be mindful of how much you eat in order to drop pounds and keep them off.

How do you know you're not super-sizing your servings without hauling out a measuring cup and scale every time you eat? You need to know how to "eye-ball" what constitutes as a serving. Most people overestimate, and that can lead to trouble. Here are some visual guidelines to help ensure that you know how to size up your food:

• A serving of meat, poultry or fish is three ounces. That's about the size of the palm of your hand or a deck of cards.
• Two servings—about one cup—of rice, pasta or other grain-based foods is the size of a baseball or your fist.
• One slice of bread is one serving.
• A serving of cheese is about the size of your thumb.
• A serving of fruit is a piece of fruit, or about the size of a tennis ball.
• A serving of vegetables is 1/2 cup, or half a baseball.
• A dairy serving fits in a single-serve yogurt container.
• A serving of fat is the size of four stacked dimes.
• One handful equals one ounce of nuts, pretzels or cereal.

Dayna Winter is a registered dietitian and food and nutrition writer in New York City.