A rough guess of what consists of a good print quality is based on the pixels density or in other words the number of pixels per inch on the printed photo. This is also known as PPI (pixels per inch) and it represents the number of pixels over an axis line.

The following qualities are generally associated with a specific PPI number:

PPI 100 - fair to bad
PPI 200 - good
PPI 300 and up - very good

The next reasonable step would be to determine how many mega pixels are needed for good quality printing on a specific paper size. Calculating this number is very easy. All you need to do is to simply multiply the page length by its width in inches, resulting in the number of square inches on the page. Now multiply this number by the square of the PPI number and the result is the number of pixels on the page which is the number of pixels we want our source photo to have. Here are the numbers calculated for some common sizes :

page 4X6 0.24MP 1MP 2MP
page 5X7 0.35MP 1.5MP 3MP
page 8X10 0.8MP 3MP 7MP
page 11X14 1.5MP 6MP 14MP
page 16X20 3MP 12MP 28MP
page 20X30 6MP 24MP 54MP

Try experimenting with different paper sizes, but be careful as this can get expensive. And remember that there are always exceptions to the rules. Sometimes what is considered poor quality pixelization can actually result in a nice artistic effect in your enlargement.

What it comes down to is that while the above calculation helps, common sense and experience are needed to match a particular digital photo to its best possible enlargement size.