[Courtesy of Oliver Wilson [ocwilson "at" sunline.net], October 1999]
Here is a cookbook approach that has some simplifying assumptions and aproximations built in.
Take 1/4 of the span in inches and multiply it by half the breaking strength of the tow line in pounds. This is the maximum bending moment at the center of the wing in inch-pounds.
Divide the maximum bending moment by the (top to bottom) spar depth in inches. This is the force at the center of the wing required of the the spar caps, in pounds.
Find the strength of the spar cap material in pounds per square inch. Most fibrous materials like unidirectional carbon fiber in epoxy and wood are about twice as strong in tension as in compression.
Divide the force in the spar caps by the strength of the sparcaps to get the required crossection of the sparcaps.
The required crossection of the spar caps decreases as the square of the distance, to zero at the tips.
The shear force at the center of the wing is half the breaking strength of the tow line and decreases linearly toward zero at the tip.
The spar caps must be attached to the shear web well enough to prevent bucking failure and the shear web must be strong enough to prevent buckling of the spar caps. Determining the shear web design to prevent buckling is beyond me. Perhaps one of the mechanical engineers on the exchange could simplify it for us.