Bela,
2000 feet left, right, and at least 1500 feet out from the flight-line. That rectangular airspace is large enough for even the highest performance Turbine Jets. Those distances are greater than anyone will care to fly... because of the limitations of most of our eyes. Depending on field altitude above sea level you'll need up to a 1000 ft paved runway the wider the better. Now if you really have the room then keep anything else from being built out another 1000 feet beyond the 2000 feet left, right. A 1000 ft buffer behind the runway left and right for a 3000 feet each way. So that would be a rectangular space of slightly more than 1 mile by about 1/2 mile.

That airspace & structure clear ground area should take care of 99% of any R/F link failure runaway Jets assuming everyone has the two second engine fail-safe properly set.

There are fail-safe settings, that if required would further insure against almost all possibility of an out-of-control flyaway... but there are very few owner / pilots that will fly a high dollar turbine powered Jet with those settings.
Lee H. DeMary
AMA 36099