It was for larger meals that "participants strongly underestimated the number of calories." (By 38% in one version of the study, and 22.6% in the second.)
So what does this mean for college dieters? Small meals do bode well for the college lifestyle. They also keep your metabolism constantly active.
But if you must have a large meal, pay attention to portion sizes. I often hear about the benefits of food scales, but even I never went that far. It's just as effective to simply visualize.
These are the rules of thumb. Source
1 oz. meat: size of a matchbox
3 oz. meat: size of a deck of cards or bar of soap—the recommended portion for a meal
8 oz. meat: size of a thin paperback book
3 oz. fish: size of a checkbook
Medium potato: size of a computer mouse
2 Tbs. peanut butter: size of a ping pong ball
1/2 cup pasta: size of a tennis ball
Average bagel: size of a hockey puck.
1 oz. cheese: size of 4 dice
Then, until you memorize the calorie content for the foods you typically eat, use a web calorie counter. FitDay is my personal favorite. Do you use another?
I like FitDay because it has an enormous food list, which you can add to if needed. You can also figure out calories burned from your activities, the calories you burn in a typical day, and track how you have progressed at your goals, whether you're trying to lose weight, stay within a calorie range, or even eat more iron-rich foods. It's definitely worth checking out.
I do have a caution. Do not use the site to enter your consumption every day. I check in every once in a while, on a typical day, to see if my diet is missing out on any critical nutrients. Or if I am unsure about the nutrition content of a new food, it's a good resource. But entering your calories every day becomes extremely tedious. Usually, I would just come up with a number very close to the one that I'd already estimated on my own, in a much shorter length of time. You will eventually become adept at coming up with a figure in your head. I know it may seem tempting to find the exact number, but it is a lot of time spent when you could be doing something you enjoy.
Hope this helps!