Skylark Promotions have developed an overland system for flag flying, circumventing the sling load requirement to avoid overflying people/property. They are marketing world wide, not sure of the web site since the Skylark one is down. Patent rights on the system, which is jettisonable, but the setup has been demonstrated in a full auto to touch down with the flag attached.
Normally a JetRanger will drag up to 12-15,000 sq ft, up to 40 kias. Larger flags need larger airframes, biggest I've dragged around was just under 40,000 sq ft, with a BK117. I'd be very careful about pulling a flag larger than the helicopter's capability, apart from undue stress, the tendency to pitch toward the cyclic limit will spoil your day!
Not much weight, but lots of drag. The cut of the flag is done to allow for the correct curve in the leading edge, so that the rest of the flag flies straight and the logo is legible. Weight at the bottom of the flag to stabilise, the line can be 1-200ft top to bottom.