In general, most cartilage piercing / upper ear cartilage / helix : a 8mm - 8.5mm inside diameter works well. This is not too snug & not too loose.
The 2nd photo shows one of my personal favorite, twisted gold ring hoop for your piercing
the 1st picture shows one of my top selling classic style ring hoop
For a snug fit or if your piercing is closer to the edge of your ear / or if you have a smaller ear ,
7mm - 7.5mm diameter will give you a snug fit.
For tragus piercing, definitely need a much smaller size, such as 6mm ( snug fit ) or go with 6.5mm diameter.
Without seeing your piercing in person, a very general guide is :
6mm - 6.5mm = for tragus
7mm = for cartilage that is pierced VERY close to the edge, so a tiny size fits well
8mm - 8.5mm = for most cartilage, rook, daith , ear lobe & 2nd ear piercings.
8mm -8.5mm round hoop is great for rook & daith piercing. You don't need a large hoop for these area otherwise it will stick out too far.
9mm = this is a slightly bigger hoop, so it is good for cartilage that is pierced farther INSIDE the ear, so a bigger size fits better.
9mm also works well for ear lobe & for nose & septum piercing .
9mm , 10mm , 11mm plus = for conch piercing.
Personally, I wear a 8mm for my ear lobe and 8mm diameter hoop is not too loose and not too snug.
Here is my easy paper strip method to measure:
measure the " how far away " your piercing to the edge of your ear / nose & then add in a tiny bit more for comfort because the thickness of the ear fold for cartilage, ear lobe, nostril. For rook piercing, measure the distance between 2 pierced hole.