It will vary from browser to browser, even in IE7 you can have two or three top menu bars and increase the bottom system tray height; some people may have a side window for History, so you can't say that just one size is available for any single screen resolution.
However, the width is the screen resolution less 18px for a scrollbar as a maximum (but even here Firefox can use the full screen resolution if it doesn't need a vertical scrollbar).
Code downloaded to my PC will be deleted in due course.
WIN7; IE9, Firefox, Opera, Chrome and Safari for Windows; screen resolution usually 1366*768.
Also IE6 on W98 with 800*600 and IE8 on Vista 1440*900.