Realistic Duna spacecraft

Hi!

I am searching for a way to build a rather "compact" but still realistic Duna interplanetary vehicle. I tried doing one on big scale (it looked a bit like the Hermes from "The Martian") but that drops my framerate significantly. Sure I could just send a normal rocket to Duna, but that would mean that I my Kerbals have to spend the whole journey in a tin can, thats why i dont wanna do that. That means I need some tips about what such a spacecraft needs to be realistic, how big it should be etc... (Of course you can include mods, I have a lot of them installed specifically for such spacecraft)

Thanks!