I am over 6" girth (circumference). I know a vagina will take a lot more than that. All the medical journals say the vaginal canal is 4-7" long, max, but gets longer with arousal.
Sorry that should read width, remember a baby can come out, so what come out can go in, and I have seen big dildos go in and I do mean big.
Barry is right. I wear a 45mm stainless steel donut ring 24x7. Balls first, then flaccid cock. And I want to add that a stainless steel ring, PROPERLY SIZED, is safe to wear 24x7. It is a static size - there is no continuous construction, so it doesn’t keep getting tighter. It’s snug enough to wear when flaccid, but barely loose enough to leave on when erect without causing problems. You can’t guess about your size if you’re going with steel. If it’s too tight, it will have to be cut off. But if it’s “just right” it looks and feels incredible.