I have used various directional wifi antennas on a variety of projects over the years, in fact this message is being posted using one on a partly obstructed connection of about 1200 ft. In my experience Yagi's don't work well if there are any obstructions at all. Best are mesh dishes, but they are relatively large 18x24 inches or so. For smaller directional antennas most of the sector antennas perform fairly well and are somewhat directional, but the best small antenna for this sort of thing is the backfire antennas sold by L-com. they are about 6-8 inches across.
