From http://docs.unity3d.com/Documentation/ScriptReference/GUITexture.html, it appears that you can use GUITexture.pixelInset to determine the size and position of a GUITexture from script
GUITextures live in Viewport coordinates. Viewport coordinates start at (0,0) in the lower left of the screen and go to (1,1) in the upper right of the screen. You can set it in script by assigning Viewport coordinates to transform.position. So to position the GUITexture in the middle of the screen:
transform.position = Vector3(0.5, 0.5, 0.0);
Note that your position will also depend on the GUITexture’s ‘Pixel Inset’ values.
Sorry to bump a months-old post, but for the sake of not starting new threads about repeated questions: