Borders should be roughly 1-3px if anything. Obv there are times when bigger borders make the piece look better but usually no.
Btw for you and all the other people just getting started or still learning. There are no rules when it comes to art. Do what looks good. That's why tutorials are so hard to make especially for better looking things because usually it's not something that you can recreate. Everything is original usually and is supposed to fit with that image.