It would be just fine for Mueller in my opinion. Mueller is 711 acres. This graphic shows how much space is in one acre. With structured, and/or off-site parking you would probably only need five or ten acres, possibly less.
http://en.wikipedia.org/wiki/Acre
We believe that to attract concerts and sporting events there is a need for a stadium with a seating capacity of 11,000 to 15,000. This is roughly the same capacity as the larger shared school district stadiums in the area (Dragon Stadium, Nelson Field) but the footprint would be smaller as the seats would wrap all the way around the playing field.
http://www.austinstadium.org/FAQ/faq.html
I think you're over-estimating the size of the stadium being proposed. It would not take up a whole lot of space.