When trying to decide what size monitor to buy the answer is always the bigger the better. You should have at least a 20" monitor. The larger the monitor, the more information you can have on your screen at one time. This makes it easier to analyze the data.
As far as how many monitors should you have, that depends on your trading style and how much information you need to keep track of. For a swing trader, 2 monitors is usually enough. For a day trader, 2 monitors is a minimum. As far as how many monitors to have above 2 is a personal choice. It depends on the information you are tracking, how many positions you have open at a time, and what you can afford. Be careful of having too many monitors as this can lead to information overload. Start with 2 monitors and then add one monitor at a time. About one every month or 2 until you reach the point of diminishing returns.
Things that are important when purchasing a monitor are the resolution (the larger the better), the contrast ratio (the higher the ratio the better), the dot pitch or pixel pitch (the smaller the better), response time (the faster the better) and the Video Input/Connectors (you want at least one Digital DVI-D)