Finding out what inner diameter you wear in nose hoops is not only really simple, but it will ensure that you get a great fitting nose ring that hugs the nostril perfectly!
The measurement that you need is the distance from the piercing hole to the edge of your nostril - this will be the smallest inner diameter that you can wear comfortably, and it will fit snugly. The most accurate way to measure your nose piercing is to use a caliper, however, you can also use a ruler if that's all you have.
To avoid the problems that come with a piece of jewelry that fits too tightly, make sure you always round up to the next whole number if the measurement you get is in between sizes. For example, if you measured a distance of 5.6mm from the piercing hole to the edge of your nose, do not buy a hoop that has a 5mm inner diameter. Instead, round up and get a hoop with a 6mm inner diameter.
Don't forget - if you're buying a beaded nose hoop, make sure you add 1mm to the smallest inner diameter that you can wear to accomodate the size of the beads that I use!