Most people follow the ‘handful’ rule. On lettuce, salad mix, shredded cabbage and spinach, most people use as much as they can grab. Which, usually turns out to be 1 1/2 handfuls. A veggie like asparagus perhaps needs to be cut up to put in a measuring cup.
Try not to fret over serving size so much. The most important rule to follow is the lean meats at 100 grams weighed raw. Just use common sense and your best judgment.