Chrome plugin to get smartphone window size [closed]
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