My flaccid measurement is about 2 1/4" (2.25") in diameter and my erect measurement is about 2 3/4" (2.75") in diameter. The cock ring I want come in sizes 1 3/4" (1.75"), 1 7/8 (1.876"), and 2". I'm not sure which one I should pick for constriction but just enough so that I can wear it all day.
I'm trying to figure out what size cock ring to get for all day wear.
The answer to this question might be an adjustable one. You can choose the intensity and the tightness yourself 
You can get the ring from the MOS shop. Buy the MOS fat tyre cock ring.