It currently hard codes it to small, but this should be theme dependent.
Created attachment 27989 [details] Adds theme method for getting scrollbar size
Created attachment 27991 [details] Fix for 24171 This differs from the last in that RenderTheme's implementation now returns a Regular size and the Mac themes return small.
Comment on attachment 27991 [details] Fix for 24171 r=me
Landed as http://trac.webkit.org/changeset/41233.