To know the size of your Apple Watch, here are the easiest ways:

1. Look at the back of your Apple Watch case. The size is usually engraved there as a number like "38mm," "40mm," "41mm," "42mm," "44mm," "45mm," or "49mm" for Apple Watch Ultra models. If the engraving is hard to read, take a close-up photo and zoom in to see it clearly.
2. Use the Apple Watch app on your paired iPhone:
   • Open the Watch app.
   • Tap "All Watches" (top left).
   • Select your watch to see the case size displayed.
3. Check your Apple Watch's model number on the watch itself:
   • On the watch, go to Settings > General > About.
   • Find the Model Number, tap it to reveal the code.
   • Use the model number to identify the exact size through Apple's support or online lookup.
4. As a last resort, you can measure the height of the watch case (top to bottom) with a ruler to match it to standard Apple Watch sizes.

These methods will help you identify your Apple Watch size accurately for buying bands or accessories.