Resizing during a session, or actual browser window size vs screen size?
It's impractical (See @JacobHume's comment below) to tell if a user is changing the window size while browsing but Chris Coyier over at CSS Tricks has come up with a way to track the window size onLoad, (results below) and zachstronaut has a similar method using Google Analytics.
Chris's article is really very enlightening, even if it is based on a small sample of CSS Tricks users. The broad conclusion is that 61% of users have their browser window within 200px of their screen size, only 1% have it fully expanded.
You could implement either script and get a fast view of the actual screen sizes your users have in a few hours, which if not conclusive, would give you a better idea than no data at all.
Should I track screen size
To add at little more detail to my reply to @JacobHume's comment below. IMHO There are practical limits in terms of volume of data gathered, impact on front end efficiency, business need and the behavioral meaning of the data when tracking window size.
Tracking every single window resize event could provide dozens of resize events per user, you could only track the start and endpoints, but the real questions are:-
- Will that screen size data tell you anything useful about your users needs?
- Will my users buy more things/see more ads/read more stuff if I support different screen sizes?
Ultimately in an increasingly fragmented mobile/micro-tablet/tablet/tv/oh-my-god-what-do-you-mean 'internet fridge'?! space all you actually need to know is that you have enough users with widely differing screen resolutions to justify (from a business perspective) moving to a responsive design.
You may then need to ask (if you can't justify the development effort to go fully scaleable) which screen resolutions you should focus your design/development time on.
Everything else is interesting but academic. The real point is that you can easily answer all but one of these questions from your existing screen resolution data.