Max width in pixels:
Max height in pixels:grab this code or pull from github
The code sometimes gets ahead of itself and you'll notice an alert indicating you need to start over - obviously still a bug - code is on github if you'd care to help fix it!
Currently this only works on FF3 and Chrome - again, github exists if you can help fix this for more browsers and/or degrade gracefully
If it doesn't work the first time, keep hitting the 'upload' button.