When you use a fixed size, NGUI automatically scales content proportionally. So let's say you specify a 1080 x 1920 interface. In that you place a 1080 x 200 pixel image. When rendered on a 480 x 800 screen, the size will be reduced to (1920/800) of the size, or 2.4x smaller. The image will be drawn 450 x 83.3333.
In your example, you probably don't need anchors at all. That interface could be achieved with a table or grid. If you want to place sprites, you can either
1) specify the anchor object relative to the previous item in the list
Sprite 1 - anchor target = UI, left = 0 px from left side, right = 0 px from right side, top = 0 px from top side, bottom = 200 px from top side (these sizes will be scaled by Fixed Size interface).
Sprite 2 - anchor targer = Sprite 1, left = 0 px from left side, right = 0 px from right side, top = 0 px from bottom side, bottom = 200 px from bottom side.
2) Place them with absolute positions and let Fixed Size scale them
Sprite 1 positioned 0,0 from top left, sized 1080 x 200
Sprite 2 positioned 0,200 from top left, sized 1080 x 200