Chrome plugin to get smartphone window size

Edit: Device Mode in Chrome 38+

Chrome v38+ has window resizing and mobile emulation in a mode called Device Mode. You can activate it either with the icon in the Developer Tools window or with Ctrl+Shift+M (Cmd+Shift+M on Mac).

Windows

As Pedro mentioned in his answer, Window Resizer will work for Chrome on Windows.

https://chrome.google.com/webstore/detail/kkelicaakdanhinjdeammmilcgefonfh

Mac OSX

On Mac OSX, Chrome limits the width at a minimum of 320px. This is the lower end of the spectrum for smartphone window sizes, so this may be enough.

If you still want to get around this limitation, open a popup window with the Open-as-Popup extension. This will allow you to resize down to 100px x 100px:

https://chrome.google.com/webstore/detail/ncppfjladdkdaemaghochfikpmghbcpc

Window Resizer will do the trick. You can even manually define a screen size.

https://chrome.google.com/webstore/detail/kkelicaakdanhinjdeammmilcgefonfh