Needs viewport resizing, too. Rated 4 out of 5 stars
It would be useful to have a checkbox next to each setting to indicate whether the dimensions are for the whole browser window or just the content area.
That way, I wouldn't have to go into the Web Console, query outerHeight - innerHeight + desiredInnerHeight (and the same for width, if on Windows).
I was developing a kiosk app that will show full-screen on a 1920x1080 monitor, so I wanted a content window of exactly that size, so that I could see how it will look.
It would also be useful to be able to assign key-chords to the settings. That would speed up testing: make a change, reload the page, and cycle through your target sizes by hitting the appropriate keys