SUMMARY Why not go further and let the user set the scaling not just by the xSize scale, but also by the distance from the screen. I think most end users somewhat know the approximate distance away from the screen more than they know the scaling they need to set. PITFALLS 1. Our eyes have a focal length to it, and I don't remember exactly what it was but objects behind objects appear in different "sizes" depending on it right? So I am just thinking, is the distance from the screen, really this thing that can be put onto a linear function, to use for scaling the screen. Of course the images need to be properly cropped for the comparison to make sense? Or maybe it is the angle? 2. One of the issues that can arise from this is users using 32" hd tvs as their monitor screen. Maybe should be a separate wishlist, but I feel it is closely related with this. I thought that maybe the user should be allowed to choose whether they are going to be sitting close or far from the screen once they connect a new screen up/connect up to a new screen, this way it is possible to quickly apply an HiDPI profile.
I also thought that the length from the screen, can be initially derived from the screen size and some sort of health/ergonomic guideline, while still keeping the option for the user to set the length to any values they want.
Eye focal length: "approximately 17mm" It is a dumb idea, but I think with a simple ruler stuck at a constant area away from the camera, it will be possible to note the size scale as the length from the object to the camera increases.
This is an interesting subject, and I have done a lot of thinking about it myself. The problem with giving users knobs like these is that most users don't understand these optical and ergonomic details. Even simply asking them the average distance they sit away from the screen will not work either since this distance will change depending on the screen content, the physical hardware, and the details of usage (e.g. with a laptop, you will use the computer farther from your face when it is sitting on a desk or a table compared to when it is sitting on your lap. You would need eye tracking to make this feature work, which would be super creepy, IMO. Ultimately I think the only UI control it makes sense to prevent to the user is a scaling slider, so they can determine for themselves what the right scaling factor is by simply looking at the screen and stopping moving the slider when things are about the right size for their preference. This also handles the case where an algorithmically-determined scale based on a given viewing distance might differ from the user's preferred scaling setting. Since we already have such a scale slider, I don't think there's anything to be done here. :)