My thought is that it is not the length that counts in the air, but the swing weight, or more precisely the
rotational inertia
en.wikipedia.org/wiki/Moment_of_inertiathe dampening effect of a mass is
proportional to the square of its distance to the center of rotation (rocking movement: the front wing of the foil).
Thus a board can still have some length for general comfort if its nose is light.
E.g.: a 5'6" board with a streamlined nose may be better in the air than a 5'0" with a big square (heavy) nose
And a 105 liters board may be much better than a 95l board if it is lighter
And if the weight is concentrated on the foil box for durability, it will better than spread on all the board...
In a nutshell: for small differences in sizing, the construction is very important.