I want to convince someone that this image belongs to the 42mm Apple Watch.
However I couldn't distinguish between them yet.
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Sign up to join this communityWhile that is just a rendering of the Apple Watch, not an image of an actual Apple Watch, the band is a Modern Buckle, which is only available for 38mm Apple Watches. So the Watch in the rendering is definitely a 38mm.
Pixel densities are 272×340 and 312x390. If the image/photo is high res enough you could zoom in to count the pixels. Or if you have the watch at home count the pixels of one detail, big ben clock, the digit 9, ...