With the falling LCD prices and seeing the that any internal GPUs can handle at least 1024x768, there is not much sense to go for a 15 inch LCD monitor, unless you are seriously cramped for space.
17 inch space is a good place to start looking for the following reasons:
1. More brand options.
2. Marginal difference between the price of a 17" LCD and a 15" LCD.
3. More real estate (aka screen space).
4. I do not think any 15" were made widescreen. Widescreen starts with 17" AFAIK.