Speaking as somebody who spent the first 4 years of parenthood driving around in a race prepared Mini (classic, not BMW model), and watching all my friends upsize to Volvos and MPVs, I know from experience that you will fill the space you have and you will also make it work in whatever space you have. We managed comfortably as a family of 4 in a Mini and we would rock up to weekends with friends in holiday houses and watch as the contents of their homes including the kitchen sink would come tumbling out of their cavernous luggage spaces. Moving to a SW was bliss, mind, and I had no regrets (having spent half my life on the hard shoulder crying rather than racing or driving the Mini) (stupidly over tuned). However, I went through 2x saloon models first before buying my first SW and I have to say that I have always found the SW, both 156 and 159 versions, not only much more beautiful than the saloon versions in almost any colour and configuration but also much, much more practical because of the ease with which you can get things in and out. The looks are so gorgeous that there really is zero compromise! You can see in the way they hold a higher second hand value how much they are loved in the market, also.
To answer the roof box comment, yes, that works. We did that (well, roof rack and zip up waterproofing) on the Mini, but found we were using it more and more so it was getting to be more of an obvious drag.
Anyway, congratulations with the arrival and all the best with your choice!