The ring inside is rubber, so it will stretch a little but still retain it's overall size. I've found it works best with cocks that have a diameter of 1 and 5/8".
In general, the angle depends really more on the cock than the harness. Most harnesses, including the Rodeoh, will leave a completely straight cock hanging down. You need a cock that curves drastically or a cock with a bendable spine (like my fave, Silky) in order to get that upward angle.
Rodeohs will stretch out over time and lose their functionality for fucking no matter what you do, that's just the reality of a fabric harness. But I would imagine that trying to use a cock that's too large would speed up that process. I supposed you could use an elastic strap to reinforce it... but that kind defeats the purpose of a comfy strapless fabric harness.