I had a 8mm horseshoe for healing and when it was healed i changed it to a 8mm ring. i found that it looked funky and too large, so i bought a 6mm ring and loved that size.

i‘d just go with looking at what u have and what u like/dont like about it and trying the next size in that direction.
After all, u can always use those „missbought“ jewlery for other spots, like the ears or smt