It looks like it would be double the width and three times the height for pak192. That means that the window would be six times as big.
I don't know where you get these numbers from. I clicked on a random building in pak64, and the window was 260*223=57980 pixels large. In pak192, that same building window would have been 328*223 = 73144 pixels large. So that window would grow by about a quarter of it's initial size.
Of course, the smaller the window, the bigger the change. An alert has 196*92=18032 pixels in pak64, it would grow to 324*204=66096 pixels, which is a grow to 3.7 it's original size - that's a lot, but a long way from six times, and only applies to the smallest possible windows, which are rare and just there to be noticed once and closed, not kept around to look at changing values.
Thus, I cannot agree with your assumption that bigger windows would hurt the game in any way. I think you can trust those who play with them on this one.
But if just changing the window size, which should be the easiest fix, is too easy for you, how about getting rid of the root of the problem instead? I'd argue that those images, iconic for simutrans as they might be, are not that useful in the first place. Unless it is a one-tile low building, you won't see it completely - not if it is multitile, not if it is a skyscraper. While you might see the leading vehicle of a convoi, you don't see enough of the surroundings to actually see where it is. Which means there are different solutions for different types of those images:
In most cases, just replace them with a goto-button, since there is nothing interesting to see, and maybe an observe mode.
For factories, replace them with a new "factory Icon", which can be a fixed sized graphic across paksets (64x64) - I intentionally say Icon, because you could use the same parameter for dats, since factories don't need icons otherwise. If no Icon is set, just show nothing.
For vehicles, replace them with the view of the first vehicle of the convoy you get in the depot, and add an observe mode
The observe mode would be the same as the image is now, but at a different scale, filling the window instead of a small corner of it. Thus, even bigger buildings and convois can be seen completely, and with vehicles you get a better idea where they are. Just like with the paksets themselves, you would see more in pak64 then you would see in pak192.
But then again, that's for an extension request, and while something like that would be cool, it's not too important. I'm happy with a bugfix