Well, actually 6 Megapixel is usually plenty. The typical large resolution monitor currently has about 2 Megapixels (1920x1200 = 2,304,000 Pixel). According to Wikipedia, the human eye has about 4-6 Megapixels(*). As long as the viewer concentrates on the picture as a whole, about 6 (though some say 3, while others say 12) pixels is enough to print pictures of any size
But of course, if you want to print a picture on the side of a house and still want to see even more details if you examine any portion of it with a magnifying glass, you will need to pack some more pixels.
Or do it like NASA: send a 2 Megapixel (1600x1200 = 1,952,000 Pixel) camera to Mars, then make huge high resolution panoramas by simply making a ton of pictures and combining them. Only works for static motives, of course, and you'll need a tripod (or a mars rover instrument arm). But with this technique, ANY resolution is possible.
(*) Thats however not a very meaningful count, it simply specifies the number of color sensitive cells in the eye. The majority of them is concentrated in a 3 degree field of view, which is also the sweet spot of sharpness of our natural (and very primitive, one element, one group lens, which however is also flexible) optics.