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.
Over time with pumping you will see gains. But I think most guys do it to see and make their cocks bigger and thicker. I myself like this and have noticed some foreskin growth as well.
I used one regularly for a while and perhaps noticed a 2cm increase, pretty sure its what also has given me a bigger head now too, it does look wayyy bigger when its in the vacuum, but even with rings once u release it it doesnt stay like it, but my normal erections are about 18cm so im not that worried about continued use of one, can make ya member pretty rock hard though in use with rings. Find the right rings n trust me you will appear thicker n feel harder!