Ok, so, basically, it's way different than the OnGUI system. You can't really compare or translate the way the two operates. NGUI is closer to GUITextures in that you have a persistent state, where nothing changes until you tell it to change - this is different from OnGUI, where you have to tell it how to look every time the method runs.
Look at how UIRoot works for NGUI, that's where you can figure out sizes. With Screen.width and Screen.height, and the UIRoot pixelsizeadjustment, you can calculate where everything ought to be placed at. I would generally advice to use anchors and set the FixedSize on UIRoot though, to keep things simple.