To set your own custom dimensions, all you have to do is load up ra2.cfg in Notepad and make a few minor adjustments. In the Graphics Config section, there are 4 lines that you'll need to change: FSDisplayWidth, FSDisplayHeight, FS3DDisplayWidth, and FS3DDisplayHeight. You'll want to set both of the width values to the same thing and both of the height values to the same thing.
Also, keep in mind that unless you have the game set to fullscreen, the window itself will have a title bar and borders that is not included with these numbers. In other words, if you have a screen that is 1920x1080 and you set the width values to 1920 and the height values to 1080, the window will be larger than your screen (unless you also have the FullScreen line equal to 1).