jayderyu, definitely a creative idea. My first idea/response for this was that a great community-effort plugin could be created and everybody here just adding their own devices. But, I'm sure this information is already out there somewhere, we just have to find it and make an API call to it.
RamPackWobble, good idea, I was thinking of something like that also. Maybe we would ask users to choose a circle the think closest to one inch (or roughly 2.54cm). Or, expanding on that, it might be good enough to just label the range of circles from easy to hard, if a game could be formed like that.
JohnnySheffield, it looks like a great website. It probably works most of the time, but didn't for me. I have a dual-monitor setup (16 inch laptop, 24 inch monitor) and the results got the pixel count right for each screen, but said both were only 13.3 inches.
Even after seeing these great ideas, I still feel that there are more possibilities out there. I just haven't thought about them yet. I'm interested in all possibilities, no matter how easy/difficult they may be to implement.