The information that you had would have been sufficient on its own to allow the dimensions of the display to be derived.
I do actually know Pythagoras's Theorem and the conversion factor between inches and mm!
Part of my irritation was that people
still insist on using imperial units. I thought that they were silly from when I first learned about them 65 years ago, and was greatly relieved when my next school only used metric units, and that was a whopping 60 years ago. Flipping heck, catch up at the back!
The other annoyance was that I had spent minutes Googling rather than seconds doing the calculation.
Remembering that I could just measure the damn thing was the icing on the irritating cake!
Anyway, I am no longer annoyed because I have done what I set out to do - produce a test version of the puzzle game I wrote, scaled to run on my Pixel phone at about the size that it would eventually be on a Pixel smartwatch. To be honest, I could have just guessed a scale factor and adjusted that until the simulated watch display was the right size! What is really pleasing is that the game is actually (just) playable at that size.