For days with less then 10kn a Stack of ARC 1510 and 1120 works perfekt (nearly 20sqm projected)
At first you think you will not be able to ride, but as soon as the apparent wind kicks in you are riding upwind well powered on a 150-160 Wakeboard in 6kn of wind, also quite nice and floaty transition jumps are possible, normal whiped jumps are nearly impossible because the windspeed is so marginal that the kites do not cross the windwindow fast enough to get them in front again.
for some pictures of stacked arcs try http://www.psn.at
If you use a large directional (e.g. Surfactory 198, lorch 205) and are not too heavy (under 80kg) you can get away with any large mid AR kite size 12 (8 proj.) or up and 40m or 45m lines and still be able to ride upwind (constantly sining the kite and therefore generating a lot of apparent wind) in these marginal conditions