Print sizes - True size of megapixels II
Image clarity is more dependant on how you shot the photo than on the number of megapixels. A clean shot from a 3MP camera is much better than a slightly out-of focus shot from a $5,000 12 MP camera.
A clear image can be printed any size from any modern digital camera. Sure, if you print mural size and look at it from inches away you won't have the sharpness you'd get from 4 x 5" film, but if you shot it properly, it will be sharp enough to look great when viewed from a distance appropriate to the size of the print.
So long as you have 100 to 150 DPI, you have plenty for a sharp print viewed at arm's length. This means a 6 MP camera can make prints 30" (75cm) wide and still look great. When was the last time you printed that big?
Ideally you'd like to print at 300 DPI to look super-sharp even when viewed too close with a magnifier. You can figure this by:
Long print dimension in inches = 4 x (square root of megapixels)
Long print dimension in centimeters = 10 x (square root of megapixels)
For example, the square root of four (megapixels) is two. 4 x (two) is eight. Thus the biggest print you can make without losing sharpness as seen through a magnifier from a 4MP camera is 6 x 8" (15x20cm). From a sixteen MP camera likewise you could go to 12 x 16" (30x45cm), and still look at the print through a magnifier.
The resolution issue is one of scale and viewing distance.
Sure, more resolution is better at bigger sizes, but how sharp your image is has little to do with how good it is. Far more important technically is whether or not the colors are correct and whether or not any sharpening was done tastefully. Many digital cameras add nasty looking sharpening that puts very artificial halos around sharp lines, making the image look obviously digital to those of us who recognize these things. Sloppy sharpening is done to impress the innocent by overemphasizing the lines around things if real sharpness and resolution is lacking.
Of course you can print much bigger, since sharpness isn't as important in color as most people worry. You can get great results from a 6MP camera at 20 x 30" if you want, since normal people view big images from further away. This is all art and in the eye of the beholder;
Don't worry too much about this, since sharpness is not as important in color as it is in B/W!
Photo credits: Andra Mohorea.