The key to getting a snugly fitting nose hoop is to get the right gauge and diameter. A hoop that is too big or a stud that is too long can be uncomfortable to wear and will cause movement between your piercing and the jewelry.
The smallest size of body jewelry that is commonly available for a nose piercing is 20 gauge. This is the standard size for most hoops and studs, though some may have smaller or larger sizes.
You can find a gauge for most hoops by looking at the inner diameter of the ring (this is what you measure with a ruler). If the hoop is already in your piercing, it is also easy to measure its internal diameter.
A caliper is a great tool for measuring the gauge of your ring. It is inexpensive and easy to use, but make sure you don't clamp it down too tight or it will scratch your jewelry!
Another easy way to measure the gauge is with a micrometer. This is a small instrument that you can buy for around $20 and will be invaluable when trying to get the gauge of your piercing.
The correct diameter for your piercing will be the one that fits most closely with the bottom of the hoop almost touching the edge of your nostril or ear. This measurement is called the "inner diameter".
The inner diameter of a hoop will be based on your piercing and your preference for how tight or loose you like your nose ring to fit. Some people prefer a more snug fit, while others want a bit of space between their hoop and their piercing.