B.M.
Until my daughter was older, we did portion size by her age. It just was easier. So.... 5 baby carrots or 5 cubes of cheese or 5 cheez-its.
I found that my daughter liked variety. So her "5" would be 2 baby carrots, 2 grapes and 1 cube of cheese. Then I would send her the little bottle of water or a juicebox.
OK - anyone who is "green" is going to have a breakdown. But for convenience and easiness we prepared all snacks on the weekend. So I had 10 (we needed 2 snacks each day) snack size ziploc bags in the bottom cripser drawer of the fridge. Each one was already separated out with snacks so in the morning all she had to do was grab 2 ziploc bags and throw them in her backpack.
If she's hungry with a big appetite though, this may not be enough food. So, I would maybe give her 2 portions at first. I wouldn't necessarily limit HOW MUCH my daughter ate - just what and when. So, she could have as many celery sticks as she wanted, but there would only be 1 cube of cheese in each bag.
If you can eat peanut butter, my daughter LOVES the thin bagels with peanut butter in the middle.
Hope that helps.
Hope that helps