A systematic methodology for conceptual aircraft design using constrained bifurcation and continuation technique is studied. The methodology uses a full six-degrees-of-freedom nonlinear mathematical model for flight dynamic stability analysis; stability and control derivatives required for analysis are estimated through standard empirical relations available in design books. To illustrate the design approach using the constrained bifurcation and continuation method, a vertical tail sizing problem of a generic six-seater airplane is investigated. Stability analysis of the aircraft lateral-directional modes with variation in vertical tail size is carried out, and a comparison of computed stability parameters is made with flying quality specifications to optimally size the vertical tail.