According to this site
I usually allow 20px for the width of a vertical scrollbar although operating systems like WIN98 SE, WIN XP and Vista vary between 16 and 18px and browsers like Safari or Opera have their own widths and then the viewer can edit his own computer scrollbar width.
Vertically it's more difficult to say what the net size is as people can add their own menu bars at the top of the screen.
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.