We are going through same thing at the moment, trying to design a strawbale house that is liveable, but not too huge. Lived in bigger homes, more to heat and more to clean.

We won't be having a formal dining or formal living room, just an open plan kitchen/living/dining area. The dining table will be huge, but will also serve as an every day table for cooking, projects etc We may have some bar stool at a kitchen bench, but we feel no need for a separate breakfast table etc. Don't see the need for parents' retreat, we will just make our bedroom that bit bigger if we feel the need to have a quiet corner to ourselves. Actually same with the girl's bedrooms, they will have room to move in there, not into tiny bedrooms and then having multiple rumpus rooms/kids retreats instead.

Some friends of ours have what we call the McMansion from hell. It takes longer to sweep the main living room than what it does to sweep my current lounge and kitchen, along with mopping the kitchen floor.