CHRIS GAMPAT, THE PHOBLOGRAPHER:
"16MP–that’s what I say most people need at most though consumers may only really need as little as 8MP. But chances are that if you’re shooting professionally or semi-professionally with the intent of only publishing on the web, you don’t need many megapixels at all. As it stands, we always recommend that folks upload their images at 1000 or 2000 pixels long to the web even as displays become more high resolution."
Read more, here.