Cooked veggies are usually 1/2 cup (say, green beans)
Raw veggies (say lettuce) usually 1 cup
Fruit? often 1 piece - say an apple. Except bananas - they're 1/2 a piece.
Diet/nutritional advice are one of the most unreliable things on the internet (after Religion & politics, of course
)
Serving sizes are really based on things like average calorie counts and grams of this or that macronutrient.
Here's an "official" list
https://www.nhlbi.nih.gov/health/educational/lose_wt/eat/fd_exch.htm
I believe that the official measures are set by the American Dietetic Association - and this gov't website uses their numbers - so I trust them more