Go ahead. Snip a site. Enter the url you want to snip into an image:

Now, enter the size you want: Custom 800x600 800x1000 800x1500 800x2000

Custom will only work if you choose "Custom" in the drop down menu above.
Custom size x (width x height)

Digg this site? Then, gimme a digg, I would apprecate it :)