I'm not sure I understand the question, could you make an example of what you're trying to achieve?
The positioning of all SceneObjects is relative to (0, 0) which is the center of the scene and if you don't move the camera, that's also the center of the screen. By default each unit of the world is one pixel large, however this can change if you move the camera's z coordinate, or if you use scaling - that is if you set the minScreenSize and/or maxScreenSize properties of your scene.
There's a short video tutorial about screen size, and one about using the camera to move around the scene. I hope it helps.