markw: I find a cockring increases my girth by a bit over .5" while I'm wearing it. I've never noticed any length increase. I go for rubber cockrings as I find the tighter they are the better the girth increase (and feeling) - and you just can't go too tight with a metal one!
A bigger ring round both the cock and balls certainly helps maintain the erection. I also like to wear one this way when freeballing as it makes the bulge look better (keeps the whole package turgid) and stops my balls packed near my cock.
Bit off topic, but I have experienced length growth through penile restriction - not cock rings but UK milk bottle! This guy I was with had a fetish of getting his buddy's cocks into a milk bottle. Normally this is difficult even for an average guy, but there are ways of doing it even for a big guy... Anyway in I went to the root, got an erection and the head of the bottle then clamped by dick to less than half its erect girth. The guy was amazed, as he could use the bottle to wank the others he had done it to... but not me - it was jammed tight. ;D
With the heavy restriction, the girth swelled way past usual, and by about 45 mins the head had hit the end of the bottle and the shaft was starting to bend
Overall I had grown over 1 inch... By that time my cock was hurting like crazy, felt like it was going to explode. Lucking he was drilling me good during that time and that was a nice distraction from the pain.
Finally we had to break the bottle to release my dick. After I finished drilling him he measured me at 9", previously I'd ever only seen 8.5.
So it can work, although it is a bit extreem!!! 8)