We have been flying flags for >10years, although only over water. One operator has approval for an overland system in Australia.
Largest that I have flown was about 35,000 sq ft. The BK was hard pushed to manage, cruise at 25kts was OK until flying back into a 12 knot headwind!
Obviously all done on a very long line, sometimes up to 300ft to the weights. As mentioned, flags are cut on a bias so that at the spec. speed the curve on the leading edge leaves the rest of the flag 'square', and able to be recognised. The whole rig has to be flown off the ground, describing an arc around the weight until it, too, is off the groud. Similar technique for landing, the flag must also be kept out of the downwash during TO and landing.
