From my personal experience it does take some experimenting to get it right and there is also natural recovery that also affects results. A tight ring can maintain the "erection" but it can make the penis cold and less sensitive. A larger and or softer ring may be less effective but increase the sensitivity. CJ put me on to the durex rings and they do work well, again two softer durex rings may work better than one tighter ring.
Google "flaccid intercourse" if you can master the technique, you will both enjoy yourselves and using the rings improves the experience. Make sure the windows are closed, the orgasm for me is out of this world. Your body is still recovering and even if you do not fully recover your natural erections thing do get better.
Someone was going to test what a recovered "erection" was actually like, i.e. does it point upwards as pre surgery or is it fully swollen but hangs down.
No doubt CJ and Lyn will be along to help.
Keep experimenting and have fun you don't say if you op was nerve sparing or non nerve sparing.