Yes, when astronauts started to go to space, they could afford a cabin so small it was useless to remove their spacesuit or even unbuckle.
But then, flights got longer and inhabited volume larger.
When Romanenko completed his 326 days aboard Mir (he had a tooth ache for more than 2 months), he only had the core module and the Kvant module to live in. Later crews could stay up to 1 year without problem (with more room too). Krikalev was told, in space, his mission would last twice as long (back in 1991) and he said: it's ok!
Now, there are issues about astronauts spending a full year aboard the ISS and NASA said no.

Mars means 3 years in space. The problem with Mars is that it's out of the question to begin a mission before results are back from a previous one. As each flight would last very long, it's going to take a long time to test. The Moon meant 8 days or 2 weeks flights, and a number of them could be included in one year.

Raoul