DPI - what size do you need

I ran across this while I was looking to see how big a 3 mega pixel camera could print. Since I've been doing the photo walking (which I did again today and as soon as I figure out where I laid my camera, I'll upload them), I was thinking about buying some cheapy 3mp that is smaller than my big 8mp. I need something that will fit in my pocket a little better.

I thought the scrappers might appreciate something like this. The image comes from the website below. :)