I seem to be just a hair larger than 1 1/8th" do I go for the 1 1/4" (which might me too big) or try for the 1 1/8"?
Without knowing your anatomy, I would say the 1 1/8". I felt like I was in between sizes and measured the larger size. I noticed that when I was fully flaccid (out mowing the grass, watching a movie, with family, etc), I wasn't touching the bars of the cage and it was causing it to slip down. I ended up reordering a new device with the smaller diameter. As Mistress MM has said, it should "fit like a glove" when fully flaccid.
If you are thinking about a jailbird, the 2 rings that make up the cage will be the size ID you order. The bars pass thru those rings so they give a tiny tiny bit more room. The cage ID can never be adjusted in size. M MM