Cockring question: first time trying steel ring

I've got plenty of silicone cockrings and ball stretchers, but I want to get a nice thick steel ring to go around the base of my cock and balls. Is there anything I should look out for or be wary of? I'm fairly average, about 5.5". What sort of size should I go for?

Thanks in advance, I'll have to post a pic when I get one ;)