Could you possibly post a link to different sizes reversing ring? I am very curious as to what got you to think there are different sizes?
As I don't know what brand of camera you're using ... I can't speak for Canon .. but Nikon only have the one reversing ring. The threaded part, which is where the lens will mount reveresed is 52mm ø. If you want to use a different thread-sized lens, then you'd need to use a step-up ring (or step-down, depending on the lens you want to use).
For argument's sake, if you're using Nikon, and want to reverse mount the 50mm f/1.8 (which is 52mm ø thread). Then all you will need is the reversing ring (there is just the one from Nikon) and the lens.