I agree, I don't think I could squeeze a mountain bike in the boot of my 4door like I could in my old 94 Bluebird.
Having no Experience with the wagons, but if in your situation, I would go straight for a StageA or even a modified R31 wagon.
(but I'm a Nissan Freak)
Edit* Just Don't do what my sister did and get a Cube thinking it will handle 2 teenagers, Mum & Dad and a Labrador.