Cool video! Seeing a "train" of those windsocks is really impressive.
I'd guess something less than 100 ft² would be just fine. In good wind I've flown my Willi Koch 'Hydroid' using an 11' Gomberg Ghost as the lifter. The Hydroid has an opening of ~5' and barely any venting, so it probably pulls the line down almost as much as those large turbines. In light wind the Ghost doesn't have enough pull, and the line makes a terribly low angle up to the Hydroid. Using my Skyform-60 as the lifter, the Hydroid barely deflects the line angle at all.

Using my 11' Ghost to lift some tube tails and the Hydroid (near the bottom right corner). The black DC is on a seperate line...