01 tacoma 02 sensor nuts

So my 01 tacoma developed an exhaust leak over the last month or so. shortly after the check engine light came on. I finally had a chance to crawl under it to see where the exhaust leak was coming from. Turns out the 02 sensor after the cats was just sitting in the port, the nuts have apparently completely rusted off? Most bizarre thing I've ever seen. Anyways, 2 questions...

1. What size are the nuts to hold the 02 sensor in?
2. If I bolt the 02 sensor back in, and have the computer reset, will the o2 sensor be ok, or do I have to replace it? I haven't brought the truck to get the code checked, I'm just assuming that's what caused the light to come on.

Any help is appreciated. Thanks.

 

I replaced the nuts with oem from Toyota, because they are made from a softer material, sort of like a pot-metal, that won't seize to the stud as they rust away, so that when you decide to replace them, you don't break off the studs and have a real problem. Just check and replace them every so often. Otherwise I'd replace them with stainless nuts. I don't know the size, probably a 12mm coarse thread.