I just ordered the 62" GustBuster umbrella on ebay . . .

That's quite large for downwind, about 20 sf: mine is 54" about 15 sf. I have found that for downwind sailing that's plenty, kayaks don't take much pushing along. It's nice to have something one can just let go of.

Some years ago I experimented with kayak sailing using a Bruce foil, which cancels the heeling force of the sail. A 15 sf sail worked very well across and downwind but it was barely able to make progress into the wind; much quicker to paddle. As the sails get bigger the consequences of a gust or a mistake get bigger too . . . so eventually I built a proper sailboat!