The holster size depends on your weapon. Determine the length and height of your gun, first. A simple google search will get you the information, or you can go through the trouble of measuring it yourself with a measuring tape. Check which holster size matches your gun dimensions. When you buy a holster online, you will see the sizes it comes in. Sometimes, the holster manufacturers specifically mention supported models. If you don’t find your weapon’s name on the compatibility list, check for sizes that fit your gun. I strongly recommend asking the holster seller instead of choosing by yourself.