Here is a nice graphic that relates portion sizes to your hand. I really like this one, because it relates the parts of your hand to specific measures rather than just associating them with foods that should be eaten in smaller or larger portions. Obviously hands vary (and the value of just going relative is that a larger person generally needs larger portions), but this is still a great method for getting more mindful about your intake wherever you are, home or away.
I always like to see vegetables on these, but this graphic is oriented more toward food you don’t want to get carried away with. You can eat pretty much any amount of vegetables and need only “worry” nutritionally and calorically about what’s on them (like dressings, sour cream, or butter).