If it is already infinite in extent, how can it be stretching or getting larger?
Not trying to be a smartass, I just can't follow that line of thought.
Power is constantly being produced in our ionosphere by the collision of the solar wind with our magnetic field. I can see two major problems with tapping into it. First, and most obvious, is just how to do it efficiently. Second, how to keep it from being overloaded and destroyed by solar flares.
I can imagine a sat in orbit trailing a tether. As it orbits it is charging batteries. When the sat reaches a predetermined point in its orbit it beams the stored power to an Earth station as microwaves. Practicality and safety concerns aside, the main question has to be how much power could the thing produce? I have no clue.
