Like other safety gears, the same elbow pads fit in the elbow. You first check the size that exactly matches your elbow size. Meanwhile, you measure your elbow size with the elbow pads. Moreover, you check by bending and straightening the arm to notice the breathability and flexibility.
If your pads are too tight it will stop the blood circulation and make it difficult to move the elbow. Meanwhile, if these are loose, then they will slip and not protect well. So carefully check the size before buying.