Hi G,
Don't forget the sim was written in the late 90's, so it has it's own limits, as the graphics were designed to be displayed on screen resolutions much much lower than is possible today. The sim has a max display resolution (as you've found), BUT as P states, if you run it as a windowed version, then Windows OS and your graphics card will drive the displayed resolution (not sim resolution), to a limit of how big or small you want to see the picture on the screen.
Personally I don't like the frame/border around the windowed version, so I always have the full screen, and 1920 x 1080 is more than sufficient to have good resolution and detail. People also get better FRs with the windowed version (as the OS and graphics card is better at rendering than the sim engine itself), even with the full screen version I use a large screen and get 58+ FPS, which is more than enough to fool the brain..ha ha !
I'd concentrate on making the sim look better with all the terrain, scenery and ac available...just my thoughts.
Cheers
Shessi