Toggle Finder sort order with AppleScript under Lion 10.7?
This is a bit hacky, but changing the sort column before and after changing the sort direction seemed to work on Mountain Lion.

tell application "Finder"
    tell list view options of window 1
        set sort column to name column
        tell column size column
            if sort direction is normal then
                set sort direction to reversed
            else
                set sort direction to normal
            end if
        end tell
        set sort column to size column
    end tell
end tell