A full aerobatic box is 3000 feet x 3000 feet x 3000 feet. Other variations are
possible.
The FAA requires a 1500 foot buffer zone around the perimeter (for jets and
warbirds, this increases to 3000 feet), so you in effect need a 6000 feet x
6000 feet footprint (or 7500 x 7500). Unless you are going to practice
cross-box maneuvers, the width of the box may be decreased.
The floor and visibility requirements are also negotiated, as are
communications and ground observer details.