The American Dietetic Association offers these comparisons as visual guides to portion size:

• A medium potato…..is the size of a computer mouse.
• An average bagel…. is the size of a hockey puck.
• A cup of fruit…. is the size of a baseball.
• A cup of lettuce…. is four leaves.
• Three ounces of meat….is the size of a cassette tape.
• Three ounces of grilled fish….is the size of your checkbook.
• One ounce of cheese…. is the size of four dice.
• One teaspoon of peanut butter…. is one dice.
• One ounce of snack foods (pretzels, etc.) … is a large handful.
  
  

Use the Portion Watch guides on CalorieKing.com

Here are two examples of the kind of information contained in the Portion Watch guides. At the end of this article, you will find a link to Portion Watch.

McDonald's French Fries	Calories	Kilojoules	Fat	Carbs
Small	255	1066	14g	29g
Medium	370	1540	20g	41g
Large	455	1895	24g	51g

Cocoa Cola	Calories	Kilojoules	Fat	Carbs
250 ml glass	108	450	0	26g
375 ml can	160	675	0	40g
600 ml bottle	260	1080	0	64g
1.25 L bottle	540	2250	0	132g
2 L bottle	860	3600	0	212g

Allow for extra calories in packaged foods. The actual weight of packaged foods is usually five to ten percent more than the label net weight (the minimum legal weight), and in some cases up to 50 percent more. However, manufacturers calculate the calories based on the net weight. As an example, the granola bar, pictured, actually weighs 130g but the packaging shows a net weight of 100g. For the most accurate calorie count, weigh the product and calculate the extra calories.

Don't eat directly out of large packages of snack foods such as potato chips, corn chips, buttered popcorn, peanuts, or chocolates. The larger the package, the more you are likely to eat. And if you don't finish the package in the first sitting, you're likely to demolish it the second time around. Either purchase small, single-serving packages or take out a specific serving from larger packages and put the rest of the package away.

When eating out, the larger serving size as well as the greater variety of foods means you are likely to eat much more. 

• Request smaller, appetizer-sized servings or half-portion entrees – and don’t feel self-conscious about doing so. You can be setting an example for others.
  
  
• All-you-can-eat buffets are a real challenge because the greater the variety of foods offered, the more you are likely to eat – especially desserts. Choose non-buffet style dining when you can, and limit yourself to two or three dishes when at a buffet.
  
  
• Self-serve drinks, whether juice, iced tea or soda, are a problem if you can refill them as often as you like – especially if the cups are large. Most large cups typically hold 16 fl. oz, which is two cups. And because restaurant meals tend to be high in salt, we can have a continual thirst. Be sure to quench your thirst with water, diet soda or sugar-free iced tea.
  
  

Purchase the smallest sizes of popcorn and soft drinks when at the movies or sporting events, (preferably diet soft drinks). An even better idea is to be well-fed with healthy food before you go to places where you know the food offerings are going to be limited to high-calorie choices.

Don't be tempted to buy extra food simply to take advantage of "free" offers. Examples include "Buy one, get one free," "Free doughnut when you buy 3."