These might help some.

http://www.articlesbase.com/marketin...ze-722349.html


http://www.cambridgeincolour.com/tut...mera-pixel.htm


With digital pix there is the problem of Aspect Ratio (particularly if you're cropping), pixel size, and DPI (which printers use).

If you're having a lab do the work, most of them have an online site where you can do the cropping and see what it will look like prior to ordering, and they'll specify their pixel requirements. If you're printing at home the charts on those sites will help give you an idea.

Hope that helps.