With boards, light wind included,
it comes down to the 3 S's:
Style: some will only want a twin, others only a directional
Shape: Curvy = carvy, straight = straight
...keeping in mind flex can make a straight line curved!
Size: For light wind the first thing to consider is size, normally.
Here are some data points on surface area:
SUP 10' long: 19ft^2
longboard 9' long ~13 ft^2 (1.21m^2)
a large alaia 6'9" long: ~9.2 ft^2 ( 0.86m^2)
~170x50 doors: ~7.6-7.8ft^2 ( 0.71-0.75m^2)
Typical shortboard surfboard, ~6' - 6'3" long: 7.5 to 8.0 ft^2 ( 0.71-0.75m^2)
Small alaia (eg ~6' x15") or Large 5'6" kite skimboard ~7.5ft^2 ( 0.71m^2)
small 5'6" kite surfboards or paipo style boards: 6 to 7 ft^2 ( 0.56-0.65m^2)
137x43 twin tip: ~5.7ft^2 ( 0.53m^2)
134x40 twin tip: ~5.2ft^2 ( 0.48m^2)
For typical 75kg+ riders, serious light wind power seems evident to me when the surface area of the board is about 7ft^2 or more.
Too much flex or rocker of course can negate this, and too little can often give good planing performance but poor handling.
Surface area over 9ft^2 is superfluous, as far as I can tell, unless the rider is very large.
Guessing at the effective waterplane area riding with an edge in probably doesn't generalize as well between board types, but on most boards I would say rarely does more than 75% of the board appear actually in the water, and often <50% at speed. Hence careful technique lets some make do with smaller boards in light wind.
High lift fins can change the game once on plane, but mostly it's effective surface area....
Then, once you figure out how big you need to go, what do you want to do out there?
I got a workout in 8-15kts on a 17 with this skim-thing, picture shows probably 10 or 11 knots:
lightwindpowercruiser .jpg [ 244.07 KIB | Viewed 1643 times ]
and more like 8 here:
P1010791sm.jpg [ 170.12 KIB | Viewed 1623 times ]
some day, it would be fun to try a directional with a retractable high lift fin, too...