If you need to span the whole screen width, you don't need to know it. Use anchors instead.
Lua Code:
UI_BackgroundWindowBackdrop:SetAnchor(TOPLEFT, GuiRoot, TOPLEFT, 0, 0)
UI_BackgroundWindowBackdrop:SetAnchor(BOTTOMRIGHT, GuiRoot, TOPRIGHT, 0, 50 --[[ this will be the height]])
edit: regarding the error, I'd say UI_BackgroundWindowBackdrop is nil (operator : is the attempt to index it). If you have that control in XML, make sure you include the .xml file in your manifest.