A spherical body of diameter D is falling in viscous medium. Its terminal velocity is proportional to:

The formula for terminal velocity is v=2gr^2(ρ-σ)/9η, where η is the viscosity. Since the radius r is directly proportional to the diameter D (r = D/2), r^2 is proportional to D^2. So this clearly shows that terminal velocity is directly proportional to the square of the diameter.