In my experience, spring peepers will not overeat. As Chudson said, the crickets cannot be longer than the space between their eyes. My spring peepers, even as fully grown adults, always preferred flightless fruit flies over crickets. I did usually offer them a mix.

In all honesty though, I would release an adult wild caught spring peeper and try to catch a baby instead. Adults do not adjust well to captivity and usually die from the stress after a few months. A baby can also succumb to stress but is much more likely to adjust to the captive environment. I'd make sure you're keeping him in a very quiet and mostly unused room in a viv with plenty of hiding places if you do not intend to release him.

As for the amount...I'd give him as many as he'll eat overnight and bowl feed so they aren't causing him stress.