It's all relative and really the only question is "is it too big for your partner(s) of choice?"
Prior to my first session, my wife was on the fence about the whole idea in terms of what it would look like... would it be odd looking... would it be too big? But she was ok with one session and gave me the go ahead. Afterwards... she was all for it. She was relieved at how it looked. She loved how it felt. She was totally on board with a second although again she was kind of conflicted, she told me to get a full session but she was concerned it would be too big. Now... we haven't used it, but she's seen it and is like "well that's a lot but I can't wait!". So it's all relative.
If all you care about is yourself (hey it's your life), then it really doesn't matter within what's possible. If you have a specific partner(s) to think about, it's a conversation you have to have. If you're single and are willing to push the envelope, there's lots of statements on the old forum saying it starts to affect certain partners.
One member did a great layout in one post of experiences at certain sizes. Maybe SO can remember, I want to say it was Sizemic. GSXR's whole progress gives feedback along the way with how his wife felt about it.
Real world... anything at 5.5
Girth is big. That's probably where a lot of those big
Dick stories you hear of come from. 6"
Girth is very large, that's where those huge guy stories are. Once you hit like 6.5 that's extremely rare... and while I'm convinced most women will gladly give that a go and love it if they're into you, if it's just a casual one time thing they might be close to their mental comfort limit/physical limit. 7" is almost not even heard of and I'd say you're past the realm of normal for sure lol.
Everything I've seen is most for casual hook ups, 5.5-6.5 is if you want to be their 'big' guy but not cause discomfort (or too much). It's all relative though. Do some digging, good experiences have it laid out.