Chastity and orgasm denial discussion

Hello! I’m thrilled to be getting my first chastity device soon. My partner has already picked out the model she prefers. Unfortunately, it doesn’t come with a set of base rings: I have to pick one from five available sizes and order an extra spare ring later at additional cost (which means it’s wise for me to nail down the right fit quickly!) I’ve read plenty about selecting the correct ring size and how snug it needs to be. What I’m unsure about is whether the tightness should feel right when soft or hard. It seems that only the largest size works well when erect, but the smallest fits perfectly completely flaccid. Since I don’t want the device slipping out while soft, I lean toward a small-to-medium option, yet I worry it might restrict blood flow to my penis and testicles during erections. Perhaps I’m worrying too much.. Experienced members, please guide me!:)