Many recipes, casseroles in particular, typically list 1/4 or 1/2 cup of chopped green peppers among the ingredients needed. Go ahead and chop up the green peppers you purchased and measure out 1/4 cup portions.
Wrap each portion in saran or plastic wrap then freeze. Once frozen solid, I store the little packets of green peppers in a plastic container in the freezer. When I need a quarter of a cup, I take out one packet. If needing a half cup, then I grab two.
Not the most original tip to share, I agree, but still a handy one to pass along ...