We theoretically investigate the dependence of the critical velocity for quantum vortex formulation on the size L of a plate-shaped obstacle moving in a uniform Bose-Einstein condenste in quasi-two-dimensions.Numerical simulations of the Gross-Pitaevskii equation reveal that the critical velocity vc is proportional to L-1/2 when L is much larger than the healing length. The power law behavior is quantitatively explained by the potential flow theory of classical hydrodynamics for the large L limit.