What is the proper portion size I should be eating at a meal?

My doctors office said 1 cup of food 3x a day. 2-3 ounces of protein, eaten first - then 1/4 cup of veggies and 2 Tbls. of a starch.

Its a guideline, but I think 1 cup is reasonable. But its very important to learn to read your satiety signals as it depends on the food. 1 cup of a lettuce based salad crunches down to nothing, 1 cup of red meat would make you explode.