It's to do with two things: What your monitor can hande, and what your graphics card is capable of.
Your card will need to be powerful enough, both in terms of its processor and memory, to handle the resolution. The load on a graphics card is defined thus:
Load = Screen pixels x colour depth x refresh rate.
You can increase screen pixels by reducing colour depth and refresh rate, although running at refresh rates of less than 70Hz is not good for your eyes, and will give headaches if you're sensitive to brightness shifts.