Rent a whole three storey mall, City Mall part of the sim-wide city of Urbana Spes. 500L$ per day, rent from just 1 day to 150 days (longer by negotiation). For pic and further details see the Classifieds and Picks on Mercia McMahon's Profile.
The mall takes up most of a 7056m2 parcel, so total floor space is more than 15000m2, an accurate measurement cannot be done due to current gridlock (asset server problems). You get 800 prims to fill with your own produce or to sub-let as smaller units. The floors are accessible by stairs and are not sub-divided.
You do not get the whole parcel just the City Mall, 300m below is a plot owned by a resident, with parcel naming powers. If you would like to have the City Mall on a plot where such powers do not exist, then a City Mall can be rezzed elsewhere in the sim. On the other hand the ground level tenant does raise the traffic (currently 484 and was over 600 before today's gridlock began). On a different parcel you could pay 675L$ per day and get the entire parcel. At ground level this includes Edit Terrain rights.
IM Mercia McMahon for further details or just pay the rental box.