I am a 5.25 but i got a 5.5. They spin around a lot I also have big knuckles. So I am probably a 5. If I had a midi ring a 4 is good right above the knuckle. Use to crack them a lot when I was young. I always thought I was a 6.25. But when I got my new set I wanted them to be as close as possible to my correct size. And I am tall 5'9 but slim.
My ring finger is a size 4.5 but only because of my knuckle, my rings spin freely. I'd try something slightly smaller but I'm afraid i wouldn't be able to take it on/off easily.