My initial chastity device. Which dimensions fit best?

It’s my first time and I want to get something to test the waters. I landed on a holy trainer knockoff but the measurements don’t make much sense to me. I’m 3 1/4 inches flaccid so naturally the maxi looked ideal, but after doing some asking I was told that a small or nano would be the way to go. I don’t see how a cage way smaller than my flaccid length would be comfortable. I don’t want to go as small as possible (nub), I just want something that fits without alot of compression. Any advice would be greatly appreciated.