In other games, like the example in the picture, more screen real estate means a big advantage in what you can select or activate. The player with the smallest screen (red) can only see a few units and will be surprised as more come on the screen.
That's a pretty good point. I remember some one mentioning in a thread a while back where he'd play Age of Kings on his 4K display. The largest map sizes could fit entirely onscreen. This could be an advantage or disadvantage to the users depending on how physically large the display is (32-inch vs 60-inch). The was we addressed the varying aspect ratio issue in one of our portrait mobile games was similar to how the Warcraft games did it. We'd use our skinniest aspect ratio (9:16) as our basis aspect ratio to draw all UI elements within. The game would be in full-view, while our widest aspect ratio (3:4) would have decorative borders on the sides. We used a fixed-height Cartesian coordinate system where the origin was at the center of the screen, and the y-axis was facing up rather than down. We'd place all of our UI elements within the bounds of the 9:16 aspect ratio, and add those borders just outside of the 9:16 view. Then, wider aspect ratios would show the border. How much of the border it showed depended on how close to 3:4 we got.
I believe we used a fixed height of 720, or maybe we used 960. In any case, this was only for placement and size (including 9-slicing). We were targeting the iPad retina as our highest possible resolution, so our fullscreen backgrounds were 1536x2048 (3:4). In a 720 fixed-height coordinate space, we get a viewport range of +/-360, which covers 2048 pixels down on a retina iPad.
This can all be accomplished with the projection matrix, as mentioned by IceCave.