I'm not sure what you mean here. What does "Closest multiple of two" do exactly? Fixed size UIRoot simply makes NGUI use virtual pixels instead of actual pixels, so if the screen height changes, your UI will remain proportional instead. So if something was 20% of the screen's height, it will remain exactly that size as you resize the screen. I'm also not sure what you mean by scaling base width and height. What base width and height? The width is determined by your aspect ratio. Screen's height multiplied by the screen's aspect ratio is your width. You can't skew your UI. You can never make a square into a rectangle by changing the width of your screen. It will always remain a square.
Unless you anchor that widget, of course.