If you only need to scroll down to the bottom of the page, there is a hotkey for that - just send "{END}" key to the IE window.
#include <IE.au3>
$oIE = _IECreate("http://www.autoitscript.com/forum/topic/124829-how-to-get-the-position-of-the-scrollbar-in-the-page/")
$hwnd = _IEPropertyGet($oIE, "hwnd")
ControlSend($hwnd, "", "[CLASS:Internet Explorer_Server; INSTANCE:1]", "{END}")