If you want to get an idea of what a portion really should be, take a look at these videos from Diabetes Care.
Breakfast, lunch, dinner:
Snacks:
www.youtube.com/user/Diabetescaredotnet#p/u/6/rqS-PLHtrLU
Revelatory for a person like me who has lost touch with what a portion of food should look like and what it means to truly feel hungry and full.