Apple watch size

Here is another thing to try. Look at the back of your Apple Watch. The size Is shown there. You may have to wipe the back with a soft cloth to clean it first. And if it is too difficult to read, do what I did - take a photo with your watch then pinch the photo bigger in order to read the print.

Is the watch paired to your phone? If so, open the Watch app, tap on All Watches and you'll see your watch listed with the size.