The inference that the Goodwood is larger than the Mars VIII and Mars IX is from the weight, 10,000lbs for the Goodwood, 6,820lbs for the Mars VIII, and 5,615lbs for the Mars IX, see Schneiderman's post above.
Regarding the Mars lineage, the Mars series was very heterogeneous, covering everything from single-seat fighters and racers, up to these transport aircraft. The only common factor seems to be the aerodynamic arrangement, most of the Mars series using the Gloster H.L.B, High Lift Biplane, configuration.
cheers,
Robin