A thought on a mkI SM: The service module would not need to be very large, given the mass of the mkI pod, nor would it need a lot of dV. Who wants to send a 1 man mission to Duna? That said, a 0.25m SM would need to be reasonably large, considering its need to have some realistic fuel capability. (Although a small SM with an unrealistic amount of fuel would be extremely stable upon reentry, given the unnatural mass/drag ratio.) (KSP needs a better aero model, doesn't it?)