Food groups -- how to tell how much is a serving?

I'm seeing a dietician for my bulimia. She has me following food groups instead of counting calories. My next appointment with her isn't until Monday... I woke up really late this morning though, and the best thing I could find in the vending machines at school was a strawberry nutritgrain bar. The papers she gave me say a serving of grains is a slice of bread, a small bagel, 1/2 cup rice, etc. How do I count a nutrigrain bar? The package said something about dietary exchanges for diabetics -- 1.5 carbs, .5 fat. Is this the same thing? Please help!!