This paper presents a novel design optimization method for fully-actuated hexrotor unmanned aerial vehicles considering horizontal and vertical acceleration. First, dynamic manipulability ellipsoids (DME) are defined and used as a evaluation index of the horizontal and vertical acceleration of the vehicle. Then, symmetric-coplanar-tilted-rotor (SCTR) structure of hexrotors is introduced, and it is proved that the DME of SCTR structure can be expressed in a simple form. Based on the analysis, a design optimization method via the DME is proposed. Finally, numerical examples illustrate that the method can handle the horizontal and vertical acceleration separately.