You say you want a big monitor, but it sounds more like you want more resolution. If you want to compare two documents side by side, a 6 foot screen that is limited to 800x600 resolution would be no better than a 15" monitor with the same resolution - the letters and images would be bigger, but you'd seen only the same amount at one time.
Conversely, higher resolution will let you see more of each document at once. I've got a 19" monitor that will go up to 2048x1536. At that point I can put two documents side by side, full size, see the entire page, and it is still readable - about the same size as a printed page.
You might want to play with the resolutions available to your monitor/video card to see if that will allow you to do what you want.