Hey pud, you make it sound like the first person camera is the same lens type for gta iv and the v trailer. They probably have a slider to control camera lenses in game. Which makes things difficault to judge. You can't assume the first person camera is for example, always a 15mm lens, that'd just be guessing. Perspective does play a part in judging a distance too. Focal length & feild of view can drastically alter the size of a distant object from image to image.
I know you coule calculate the height of an object based on real world object scales, but that's still not accurate. I'm not dismissing your efforts, as we can all see just by watching the trailer and looking at pics, that it seems like a fairly small city.