My longtime girlfriend is off limits, pal.

If you don’t plan on marrying her very soon after, it might be.

If you want to surprise, measure it with a length of yarn/string/whatever while she's asleep.

I wouldn’t

Honestly you might want to find out if she would prefer to pick out the ring. I know I would

I took one of ex wife’s rings to a jewelry store and told them to size the engagement ring I bought to that. Same with the ex fiancé. They both had 4 or 5 they regularly wore, but not at the same time. They left for work, I grabbed a ring and went shopping.

Okay...wincing a little here. I'm no good at this but, asking is a dead give-away isn't it. If you're buying any sort of ring, I'd go to the jeweler I was going to use and ask about the ring I had in mind, and if they could size it after the fact. At least, that would be my thought.

got to be a better way of finding out, does she ever wear a ring on that finger on the other hand?

I knew before my husband proposed and it didn’t make anything less special 🤷🏼‍♀️. I even chose my rings because he had not a clue! I think it’d be ok to ask if you’re both gonna be alright with it.

Not if your doing it just because you think it's what you're supposed to do. You should get married if that is what you both want.

Just ask her. If it's easier take her shopping with you and let her try on random rings.