I have a two year old, and both her and all her gubbins fit in it just fine. It'll happily take 4 adults and I'm just shy of 6 foot.
The boot isn't the biggest, as there's a big diff and subframe under there, but it's good enough. However, I also have an A6 Avant - so I don't have to rely on it being the only car for the family.
3 doors are doable, but a pain the the frigging arse, and why I got rid of my 135i coupe - fine when the kids are being compliant, but it's like trying to get a cat in a bathtub when they're not.
It's an amazing car for what it is and what it costs, but in your situation, I don't know what the best answer is?