You only "need" the cheapest monitor you can find. If you're willing to pay the extra amount, I definitely recommend a 19" or bigger, it makes a big difference.
I am now using a 19" and as Fatburger said, I definitely find an improvement over my 17". What kind of budget do you have? Are you willing to spend the money for a 19" monitor?
Rob
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if your on a budget then you should probably get a better 17". while 19" is a lot nicer, if you can only afford one for the price of a 17" then your gonna get what you pay for.