The ISS design is ultimately constrained by cost and fairing size (how big a thing you can fit on your rocket), limits that are far more stringent for the surfa

The ISS design is ultimately constrained by cost and fairing size (how big a thing you can fit on your rocket), limits that are far more stringent for the surface of Mars (where the limiting size is set by the descent stage). The fact that it's in vacuum doesn't affect its size at all. The average surface pressure on Mars is a few millibars; from an engineering perspective it's the same as building pressurized structures in vacuum. If anything, it's more difficult, since your design will have to deal with wind loads and airborne dust.