im guessing you mean how much of the screen is being taken up with the actual content of a webpage as opposed to the content with the toolbars and suchlike.
the size in pixel that is being take up would depend on what resolution you have your monitor set to.
the easy way to find would be to set your monitor to different resolutions and take a screen shot and select the area you're interested in using a graphics package like paintshop pro and look at the size of the selection you've made