Southcross

This is kind of a first for me, so I'm going to have a lot of questions

I was given an old 1970 Schwinn Super Sport (w/ 27" tires) that needs rebuilding... a lot of parts were missing or non-func. My friend Frankensteined it out of a couple of other vintage bikes to make it mostly operational, and one day I'll worry about the non-original parts, but "today" I'm trying to source brakes... I have a damaged front brake and no rear. Searching ebay I see a couple of different size brakes (610,730,750). How do I determine the correct "size" brakes for my bike?

edit... further re"searching" here yielded a link to a website with details on how to measure brake reach on the caliper and the body of the bike, so I do have something to go by. But I'm still wondering if maybe there is a resource that would say specifically what the original part was.