I beleive that to do full engine runs a B777 that it needs to be fully loaded with fuel other wise it jumps around too much. Thats the power of the big engines.
With those fitted to the 737NG I beileve that the saftey distance is 2 Meters? at idle power and 5 meters at full take off power.
Hay Ewe - KEEP OUT OF THE WAY!!!!