240 x 320 (small phone) 
320 x 480 (iPhone)
480 x 640 (small tablet)
768 x 1024 (iPad - Portrait | Small desktop monitor)
1024 x 768 (iPad - Landscape | Medium desktop monitor)
1200+ x anything (Large desktop monitor)
A tool by Matt Kersley - Fork it on Github
Note: The content width may not be pixel perfect - I have added 20px to the iframes to cater for the scrollbars