I believe it is 23px. I never make a site wider than 1000px.
Change your theme in Windows to high contrast extra large and you should get the biggest scrollbars you can get.
The scrollbar is 15px here on OS X.
Also, check this out: browsersize.googlelabs.com/… Google’s viewport size stats. As you can see, 960 is the safe spot and you could get away with 1000, but it wouldn't be perfect.