Finding the (ideally) safe landing zone radius for hypothetical VTOL transports is a little hard because there haven’t been that many of them. The first precedent is the ideal landing radius for a V-22.

From a Marine Document, we get:

V-22s are about 84 feet wide at their widest. So that ranges from 2.1 times their width/wingspan to 4.1 times.

The second is an EASA draft regulation on “vertiports”. The “D” value is defined a circle around the aircraft when its thrusters are in takeoff/landing mode.

The draft describes the safe landing area as at least 1.5 times the D-value of the aircraft plus a safety buffer of at least 3 meters or 0.25 times the D-value, whichever is greater. These are of course just guidelines (and keep in mind they’re for constant civilian travel, not military action), but they’re still good rules of thumb.