How selective should I be in giving referrals?

Is it true that if you refer too many "bad" candidates, you'll get a bad reputation? What would the consequence of this be and what's the blast radius?

Yes. The recruiting team will start hating you. If it gets really bad, I can see it escalating to your manager.

How do referrals in big tech differ from mid-sized companies? I'd imagine the smaller the company, the more your individual reputation matters. Does reputation still matter in big tech?

You'll get punished more for bad referrals the smaller the company is as it's more tightly knit. At Big Tech, many people run referral farms where they openly ask for strangers to refer. These companies are so massive that they don't have the time to keep track of who's funneling in mediocre referrals.

What would be considered a "bad" candidate? Let's say I refer a friend who passed the resume screen but failed the recruiter screen, would I be liable to sourcing a bad candidate?

It's hard to draw a line, but I think someone who can't pass the recruiter screen isn't great (of course, someone who fails the resume screen is even worse). I would want most people to make it to phone screen at least.

Can you rank the scenarios in terms of what damages my reputation from most to least:

These are weird scenarios that are almost certainly never going to happen haha. Just refer good people you know and don't worry about this too much. I think #1 and #2 are fine. #3 is stupendously unlikely, but that's more of the hiring committee's fault, not yours.