Strictly, to be full-screen for everyone you should go from -1.01 to +2.01, since the greatest width that ArmA supports is three 4:3 monitors, arranged horizontally, which gives 12:3 or 4:1 ratio (more might be possible by tweaking, but I'm not aware of that possibility). This means that the potentially visible area of the screen should run from -1 to +2 horizontally (though it is wise to make things a tiny bit bigger so you don't get a visible "edge"). As you can't tell what format the player is using, only put information/interaction elements inside the regular 4:3 (0 to 1 horizontally) space. Outside this area, just use a "flavour" image, like the speakers on the BIS TV, or just fill with your background colour if you don't want some people to be able to see the 3D world around the edges of your dialog.
EDIT: Remember that if you have less than a 12:3 screen, which most people don't, you can always set ArmA into that aspect ratio for testing purposes. The screen will appear severely squashed, but at least you can see what your dialog edges would look like to every player.