It's super important to make sure that the case or protector you purchase will actually fit your Apple Watch, and the best way to do this is to first find your device model number and case size by checking the engraving on the back of your Apple Watch. This should tell you which model you own, but you can always Google the model number to be totally sure.
Now you know which Apple Watch you have, be sure to read carefully on any  Amazon listings or product descriptions to see if your watch is compatible, or if the website isn't very helpful then you can always measure your watch and get the exact dimensions and compare this with the product before purchasing to know if it will fit – or if this sounds like too much effort, you can always just contact the retailer and ask them directly.