We have found that 8,000 square feet seems to be good. Full size field, plus room for the various subteams to work with relative quiet (from the noise generated by the mills, hammers, etc), room for material storage.
Of course, robot teams will expand to fill the available space.
Twice in the last three years we have been in a local mall with a lot of available space. Free the first year, $2500 this year. But it really makes the team productive, not having to put away everything after every build session.
We had several teams show up to use our practice field when they could, which benefited everyone.