AppleScript for window resizing (2 monitors)

Does anyone out there know of an AppleScript (or similar) that will allow one to input a percentage of screen width or a number of pixels and get a window to resize to that measurement? I gather it’s tricky to achieve with an external monitor hooked up. I’ve used Breeze, Divvy and Sizeup, but I still miss the direct method of typing in a size and getting the window set to that. Partly, I don’t like reaching for the mouse every time I want to resize.

Cheers,
Martin.