A conceptual design framework is presented for studying how the mass of a large space telescope mirror will depend on design disturbances, mirror diameter, and practical structural design constraints. A variety of on-orbit, launch, and ground test design requirements are considered, as are practical constraints on structural truss member properties. While prior work emphasized the trade between structural depth and overall mass fraction, this paper shows how these practical constraints limit the achievable structural depth, and thus define an optimal depth. An example of a tetrahedral support truss for a segmented mirror is presented. For lightly loaded design cases, it is observed that the minimum mass structure is determined by the simultaneous application of minimum allowable tube thickness, a specified strut Euler buckling load, and a specified strut pin-pin frequency. Closed form solutions are derived for the optimal structural depth and areal density. These are shown to be independent of the diameter of the telescope mirror.