Tue Jan 20, 2009 5:49 pm
This all depends on how large you want to print, what processing software you are using, what the distance is for the viewer of the print and what print quality you are chasing.
Lets break it down...
Say that you are chasing a print standard of 300dpi. A 4x6 will have a dimension of 1,200 pixels by 1,800 pixels (just at 2.16 MP). A 5x7 weighs in at 1,500 pixels by 2,100 pixels (3.15 MP but this is a slight crop of the 3:2 ratio so 3.4 MP is needed). An 8x10 is 2,400 x 3,000 (7.2 MP but keep in mind that you have to crop 8.3% off the length to get this so 8.7 MP is needed). If we want to bump that up to a 16x24 print it will have a dimension of 4,800 pixels by 7,200 pixels (just under 35 MP).
It's obvious that many photographers print much bigger than this with cameras in the prosumer range. Heck, I've printed 16x24 from my Canon 1D MarkII (8.2 MP) with success. There is software (and companies that do this) that is made for huge renderings of images (billboards, buses, posters, wall art, etc).
If you do this for a living and deal with the media (print), certain minimum standards are required. If you do this as a hobby, experiment. In dealing with outside print services, make sure you know what their profiles are. Color variations in printing can be all over the board. Know how to set the embedded color profiles that match the printers for the best end result.
I hope this helps a little. I'm guessing a great deal about the specifics of your question.