Figure out the initial mass in LEO required for a human mission to Mars.
Huge dependence on number of astronauts, what kind of orbit you enter, what kind of ECLSS, what kind of space you think they'll need, and mission mode (long-stay, short-stay, fast-transit, etc).
For minimum, I'd say look at what it'd take to push Salyut 7 (just the main module, plus an attached Soyuz) through the delta-v of barely entering Mars orbit (perhaps with slight aerobraking), then leaving Mars orbit ~500-600 days later for trans-Earth insertion. 1kg/astronaut (very calorie-dense food, efficient ECLSS) for 1100 days for 2 astronauts. And lets say the whole stack starts at EML2.
In that case, there's about 1km/s until trans-Mars insertion,
1km/s .9km/s for Mars capture,
1km/s .9km/s for Mars departure (and reentry at Earth) for a total of 2.8km/s delta-v. Assuming something equivalent to Block-D (stretched, having a dry mass of 4000kg), maybe slightly better (EDIT:hypergolic), we have an exhaust velocity of
3.4km/s3.2km/s.
Soyuz is ~7000kg.
Consumables maybe 2000-4000kg (we need some margin here, 1kg/day is pretty optimistic...).
Salyut 7 is 20000kg.
Dry stretchedBlock D used for all large maneuvers: 4000kg
Total: 35000kg.
35*e^(2.8/3.2)=~85.
So, about 85 tons for two astronauts at EML2. Obviously much more for the actual IMLEO, but that's dependent on propulsive technology used to get there. 85 tons isn't bad.
EDIT: I think full hypergolic would be better than using kerolox, simply because we know how to store hypergolic fuels for decades. To counter-act, we can assume .9km/s each way into/out of mars capture, to get basically the same total mass at EML2. I had forgotten at first that Blok D was kerolox.
