How do you add a box collier of the same size of the GUITexture?
You cannot. GUITextures are 2D constructs that live in Viewport space. Viewport coordinates start in the lower left of the screen at (0,0) and go to (1,1) in the upper right. The texture size for a GUITexture is specified in pixels (like are used in Screen space). Colliders live in 3D, World space. It is possible to have an empty game object in world space that tracks a GUITexture. But if you wanted to dynamically size the collider to fit the GUITexture (rather than set it by hand), there would be a number of Viewport and Screen conversions to do. It would be easier and better to replace the GUITexture with a plane in world space.