there are some good guidelines out there for serving sizes:

a serving of meat (chicken, steak etc) is a deck of cards or the size of your palm.

cheese is your thumb (or a pair of dice)

most fruit is a tennis ball

rice and pastas are about 3/4 of a cup. (COOKED!)

most bagels are 2-3 servings of grains, a full pita is two servings.