If you’re showing graphics in a graphics window, then you can set the window to be of a maximum pixel size so that it will never be stretched larger. But you’d also want to set a maximum proportional size so that it doesn’t consume the whole game window if the window is small. See Counterfeit Monkey for an example of how this works. I don’t remember if this functionality was included in Flexible Windows by default yet; it may need some extra code.
But that said, this doesn’t account for higher pixel densities. I proposed a Glk extension to expose pixel densities, but there hasn’t been any interest for it yet. You could just fudge it by always using a double density image.