As big as I can afford to pay people to clean
Our house right now is about 2100 sq ft main and upper floor, with an addition 1500 in the furnished basement, and it's definitely not too big. We could use more storage space for big stuff (don't like to keep the track tires in the garage over the winter). But we don't feel cramped.
Personally, I like big open rooms, but I don't like "open concept" layouts. I like a room for a purpose - a separate dining room, a separate den, living room, etc. but I still want them spacious.