How big can one gauge their ears before the whole won't close back up to "normal" earring size?

It depends on your skin's elasticity, if you don't stretch too quickly, if you don't skip sizes, etc. Stretching is viewed as a permanent modification since there's no guarantee you can reverse it!

I know a few people who all used to be at 5/8" and all of their holes shrink/closed when they wanted them to.

EDIT-- And their ears DID look normal afterward. Please don't talk about things you don't know much about, person below me.

Most people feel 0 gauge is the largest one can go without returning to a "normal" size.

Any larger than that and they will never return to their original size.

To the poster who stated that they knew of people at 5/8ths who stopped stretching and returned to their original size; they may have been able to put the jewelry in-- however it's impossible that it would ever look the same.