You could just carry a paper plate or a plastic plate for a week or two just to get the approximate sense of what is a reasonable amount.
And/or, just start looking at your plate of food at meals where you do use a plate. Get used to the proportions.
My rules of thumb for lunch or dinner are about a palm-size portion of protein or a fist of beans, approximately two to three fists of starch, a thumb or two of fat, unless there's a lot in the dish I'm eating, and several fists of freggies (more veggies than fruit). These days, one of those two meals is about 2/3 the size of the other. If I am eating out, I also cut down the volume a bit because the food is likely heavier than I would make. Then again, I've been out of college probably twice as long as you've been alive, so you might need more food.
Do you not have access to any kitchen? You could assemble simple meals for WAY less than you are spending on "grabbing" food out. (Slim cultures do not grab food.) A divided plastic container would cost you less than a deli sandwich.
I know you didn't ask for this but this excerpt might give you an idea. (This is about simple, cheap meal prep for those who don't have a lot of time and are willing to eat rather simply for the time and money saved. Not me now, but it did seem very practical.)
http://earlyretirementextreme.com/cooki ... han-4.html
Count plates, not calories. 11 years "during"
Age 71
BMI Jan/10-30.8
1/12-26.8 3/13-24.9 +/- 8-lb. 3 yrs
9/17 22.8 (flux) 3/18 22.2
2 yrs flux 6/20 22
12/20/24 24.1
There is no S better than (mod) Vanilla No S