If you're asking about the shielding, probably the mass required for materials that are generally used for radiation shielding. If the craft is built terrestrially, the amount of energy necessary to launch would be insurmountable with current chemical rockets.

Now, if the craft were manufactured in space (and forming of the shielding materials were practical in low-G), the problematic materials could be shuttled up over time, making it a non-issue. This would, of course, also mean that the craft could not be used for re-entry and would require landing craft. And there's all the logistics challenges (supplying air, etc). Probably though the direction that will be necessary for long-distance space craft.