Everybody's anatomy is different of course, but I think 45mm is more common (or at least closer to the median) than 50mm, as it's usually the middle of three sizes offered. It would be strange to me if the biggest size offered was too small for someone who describes his cock as small. Not saying you're wrong about your own situation if course, just that maybe you still need to get used to putting on and/or wearing your device.
For me, I have relatively large balls (I tell my wife that my dick only LOOKS small, because it's right next to some large balls, but she doesn't seem to buy it!) but I still use 45mm rings. It only has to be big enough to squeeze one ball through at a time. Once I have it on, the snugger the better. Otherwise it can slip, chaffe pinch, etc. In my experience, a device should be as tight as possible without restricting ******* flow.
Basically, I pull a bit of my scrotum through the ring and push one ball through the ring into that, then repeat for the other ball. I don't really know how to describe getting your dick through after that... I kinda roll the skin forward to retract my shaft, then put the tip near the top of the ring and "unroll" myself through the ring, if that makes sense at all. I admit, it doesn't sound like a very good description as I reread it.
So I don't know if that's helpful or not. I will see if my wife will get a short video clip while I put my cage back on after my next release, if that would be helpful to you.