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Using a javascript event triggered by the onload event, this plug in measures the browser viewport size of your visitor's browser and sends the result to your Google Analytics account as an event.

You must have the asynchronous Google Analytics tracking script installed on your site and use _gaq as the object name. The template you are using must support the body_class filter hook introduced in wordpress 2.8

Details on the technique behind this solution can be found here http://beantin.se/post/354097397/track-viewport-google-analytics

To view the results, log in to your Google Analytics account (but wait a day for GA to process the data) and look at Content -> Event Tracking. In that report you will see and event called "Viewport" with three actions; "Width", "Height", and "Size". The label within the action "Size" shows the exact dimensions of the browser viewport size of each visitor to your site.