Apple Watch covers
How do you know what size screen protectors to order for Apple Watch?
Apple Watch Series 7, watchOS 9
I'm assuming you are referring to third-party watch screen protectors ... correct? All of the Apple Watch specifications can be found here: Apple - Support - Technical Specifications
If so, then you would need to order the one for the exact Apple Watch model that you are getting. You may also want to know the store's return policy should it not fit properly.