Serving sizes are sometimes a mystery. Especially when eating out! When I eat out, I generally assume most of what I am eating is 2 servings. One trick is to ask for a "doggy bag" when your food is delivered. Immediately put half of the food in the box to have for lunch the next day.
Another trick I do is to split an entree. One time, Michael and I went to one of our favorite restaurants for lunch, Papa Haydn's in Sellwood. We got the Croque Monsieur (Black forest ham or turkey on grilled parmesan coated bread with gruyére and dijon) and split the sandwich. We also ordered a caesar salad and split that. It was the perfect serving size for us!
When eating processed food, pay attention to the labels. I found a nifty guide online (see above left). Sometimes there's multiple servings in an item and you don't even realize it!
Here is a general guide on serving sizes:
Meat or Poultry (cooked)
Suggested portion: 3 - 4 oz.
Size guide: A deck of playing cards or a computer mouse
3 - 4 oz.
A checkbook, or an eyeglass case
Two cupped handfuls
Veggies (raw or cooked)
Roughly the size of a baseball
1 medium piece
1 small slice
A cassette tape
Pancakes and waffles
2 small pancakes or waffles
A compact disc
A small potato, about 3 oz. (most restaurants serve 6 - 12 ounces of potatoes!)
A bar of soap
Dried fruits and nuts
1 1/2 oz.
A large eraser or 4 stacked dice
Yogurt and milk
1/2 a baseball, or a Clementine
Small package of dental floss
Oil, mayonnaise, and butter
The tip of your thumb
*Most people use far more butter and oil than the standard serving size, 1 teaspoon. For reference, 3 "tips of your thumb" measurements are equal to about one tablespoon.