I’ve been taking a Nutrition class as part of the prerequisites for the various Nursing programs I am looking at applying for next Spring. My professor posted a great PDF from WebMD that is a portion sizing guide. I almost always over-estimate portions. I think most people have the very same problem. They have wallet and fridge size for reminders both at home in the kitchen and in restaurants on the go. I hope it helps you out, too.
Wallet Size: http://www.webmd.com/diet/printable/wallet-portion-control-size-guide