A helpful guide if you're stuck on whether to purchase the 38mm or 42mm size

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Here's a helpful guide from iMore that may assist you on whether to purchase the 38mm or 42mm size Apple Watch. Also, you can download the Apple Store app on your iOS device, browse to the Apple Watch section, and click on the case options link to view real sizes while holding it over your wrist to get a better idea. Hope this helps.

Also, this site has some helpful cutouts that you can print out and wrap around your wrist to see which works best for you.