The question involves Area and Aspect ratio.
From the Lift Equation we have:
Weight = Lift = Density × Area × Velocity$^2$ x Coefficient
Coefficient is Angle of Attack of a given airfoil. This data can be found at airfoiltools.
If you know your weight, design airspeed, fluid medium density (generally of air) and Coefficient at optimal angle of attack, Area can be calculated.
Sink rate (for a glider) will determined by the amount of drag produced at a given velocity to make Lift. Drag is equal to the amount of forward force (provided by gravity) to keep your velocity constant. This force is readily obtained from the glide ratio × the aircraft weight or can be deduced from the Lift/Drag ratio of the airfoil plus the form drag of the aircraft.
So now you test your sink rate at various airspeeds to build your sink rate vs velocity curve. We can see induced drag will be greater at lower speeds, and form drag is higher at greater speeds. The lowest drag value for the sum of the two is Vbg.
Too small a wing means too much speed is needed, too large is simply adding extra weight and drag.
If you get this far with a good strong rectangular Aspect Ratio 8 wing, glide ratio can be improved by increasing AR to 10 or more, at the expense of strength. This is more fine tuning. Gliding birds, such as albatross, have high AR, whereas acrobatic birds, such as hawks, favor lower AR strength.