Square footage is important, but don't forget to think about layout! That is sometimes more crucial than what the numbers say. If the house isn't laid out in a way you can use it functionally, it doesn't matter how big it is. From my experience, anyway...
We are a family of seven in a 00 sq ft house. The house is almost a split level. Its mostly one floor, but there is a half flight of stairs going up and down at the far end of the house. Upstairs has two bedrooms, a large hall closet and a full bathroom. Downstairs is a family room that leads to a smallish one car garage. The space is perfect, though I'd prefer to convert the garage and family room to two more bedrooms - then everyone would have their own space, and the whole house would be used. Right now, we only use the main living room.