Screen captures of windows larger than screen Is there a way to capture a screenshot of a browser window / webpage which is larger than the MacBook Air (1366x768) screen it's displayed on?

I need to have captures of how a site displays in a 1280x1024 window. I can use Moom to resize the browser window, but not sure how to get the snap.
You can take screenshots of entire windows, even those that extend beyond the bounds of the screen, by pressing ⌘⇧4, then space, then clicking the window.