When people first think about digital cameras they immediately think about how many mega pixels the camera should be. Why a minimum number of mega pixels are needed to achieve a quality print, there is much more to picking a digital camera than just the number of mega pixels the camera records.
With that said, mega pixels are still a good place to start our discussion. Observe the chart below that shows the relative sizes of images produced by various sized sensors.
While people will argue over what the minimum resolution required to output a quality image, I have found that starting with a 200 dpi image and boosting it a bit in Photoshop gets acceptable results. Ideally, you would like a dpi of 300. Using the 200 dpi lower bound, you can determine how many mega pixels you require based on the size of the image you wish to print. Even these estimates are on the safe side, with some photoshop tweaking you can go larger.
Something else to consider
Of course there is a price to pay, both figuratively and literally, for using a camera with a large mega pixel capacity. The higher capacity sensor the lower number of images you can store on a memory card.