This can be frustrating for most so doing a blog about it seemed like a good idea.
Let's work with the right diameter first. All body jewelry is and should be stated as the inside diameter of the hoop. What you need to do if find yourself a ruler. Using the mm part, place it from your piercing to the rim of your nose. There's your diameter. That will give you a comfy fit without the hoop hanging down. Many like to have a hoop that's a bit larger so just add 2mm to that to give you the diameter you want.
Getting the right length for a stud or bone is super easy. Ninety five percent of folks need a 6mm length. Once in a while someone has a super thick nostril so in that case I would go with a 7mm bone or stay with wearing studs since you can adjust those rings.
Figuring out what gauge you need for nose studs/bones is a bit more complicated. I'm a piercer so I'm very accustomed to what sort of jewelry my clients will be wanting to wear once their piercing heals. I pierce in 20 gauge since most of the nose bones and nose studs come in either 22 gauge or 20 gauge. If you were pierced with a gun you are most likely 20 gauge. Most piercers that aren't familiar with the different jewelry out there will pierce in 18 gauge. No worries though because you can easily get your nose to take 20 and 22 gauge.(as a side note, the larger the number the thinner the post so 22 gauge is thinner than 20 gauge) If you were pierced at 18 gauge but want to go thinner you need to keep in mind, with your first purchase that if the ornament is too small on the ring then it could very well just sink into your hole. What you need to do is pick an ornament that is at least 2mm. That way it is bigger than your hole. After wearing this for a few days your piercing will shrink down to your new thinner gauge and fit much better. For the first few days I'd recommend wear a bandage while you sleep so that your ring doesn't fall out and you should always have an extra ring around in case you lose one. Nose hoops are a it different. I pierce those in 18 gauge because if it's too thin it could rip your nose while it's healing. This gauge isn't very thick so some choose to stay with it. Most end up switching over to 20 gauge when their piercing is healed because they like the dainty look. Definitely do that if you prefer dainty but please be sure your piercing is healed or you could end up taring it.
Let's say you are use to wearing 22 gauge but see a ring you really like that's 20 gauge. I'd go ahead and buy it because there's very little difference between 22 and 20 gauge. All I would do extra is add just a little touch of oil (any kind) to the tip of the post so that it slides in easier. Now bouncing from 22 gauge to 18 gauge is a bit different. It's probably going to be a bit sore. I do it all the time but I also do my own piercings so a little bit of stretching ...meh. If you are going to do this and want minimal discomfort I would do it right after you've had a nice hot shower or bath. Your skin if far more elastic and willing to open up. You may feel a slight ache in your piercing the next day but no worries, it will pass in a day or two.
I hope this has helped you some but if you have any questions please feel free to ask away.