Browser Window Stats is a WordPress plugin by ChemicalSailor that records browser window size for each page load and makes data available for administrators.
Data is stored on a per visit basis, which contains each visitors page views. A visit times out after 15 minutes of inactivity (no more page loads).
Visits made by registered users are also counted, and data is collected for page views in the admin area (your own visits are important too!) except for the plugins own stats page.
Download and info: http://wordpress.org/extend/plugins/browser-window-stats/