Manufacturers publish this kind of information in aircraft characteristics documents, as it is useful for airport planning and operational safety. You can find the A380 document here.
The document lists the exhaust danger area at max takeoff power. For the Trent 900, this extends 1800 ft aft of the nozzles, and the GP 7200 extends 1553 ft aft. Anything not bolted down should probably be removed from this area.
The document also lists exhaust velocities. At max takeoff power, for GP 7200 engines, the exhaust velocity exceeds 105 mph out to 724 feet, and 65 mph out to 1090 feet. Higher velocities are not labeled.
On Physics.SE, it's been calculated that 45 mph is enough to move a person, and 70 mph could start to overcome gravity (depending on orientation). So if you don't want to be blown over, it's probably best to stay out of the exhaust danger zone.
This paper suggests 95 mph is enough to tip some high profile trucks, trailers, and buses. 200 mph is enough to tip most cars, vans, and pickups. If the vehicles are light, much less than this could be sufficient.
Based on this info we know that at full power, large profile vehicles could be tipped over even at almost 750 feet behind the aircraft. Aircraft designed to fly at slower speeds or lighter aircraft could easily be picked up by these high wind speeds. Tornadoes have been known to move aircraft as large as a C-130.
Jet aircraft generally do not go to max takeoff power unless they are starting their takeoff. Engine speed is usually limited in ramp areas for safety. For engine run ups, the aircraft go to dedicated areas with jet blast deflectors.
Even at ground idle, the exhaust danger area extends 230-280 feet aft of the nozzles.
You may also want to consider temperatures, which can exceed 212 F up to 100 feet aft of the nozzles at takeoff power.