The two different measurements that you need to know before buying tragus studs online are: 1) the gauge that you wear, and 2) what length of post you need. The easiest way to obtain these measurements is to simply ask your piercer, but if that isn't an option you're not out of luck.
Quick Note: Traditional 20 gauge post earrings intended for an earlobe piercing are not likely to work well for your tragus piercing. Not only are they usually too thin, but the post is also going to be too long to fit comfortably in your ear - most stud earrings will have a post that is between 10mm and 11mm in length.
To find out what gauge you wear, simply use a caliper to measure how thick the stud is that you are currently wearing - the two most common gauges used for tragus piercings are 18 gauge (1.0mm thick) and 16 gauge (1.2mm thick). You must use a caliper to measure the gauge, a ruler will not work for this step.
Next, you can move on to finding out what length you wear. There are two ways to do this - you can either measure your actual piercing, or like you did with the gauge, just measure a piece of jewelry that already fits comfortably.
This is a really easy measurement to take! In a straight line, measure the distance from the piercing hole to the edge of the cartilage - this will be the smallest inner diameter that you can wear, and it will fit snugly. If you are going to get a beaded hoop, make sure that you add 1mm to this measurement to accommodate the size of the beads that I use.