8000 sq ft on 3 levels, 1.5 ac lot. 3 kids, 2 parents, 2 grandparents. We used to live in a 3400 sq ft house and managed but always felt it was tight, some of it because of the distribution of space. Also lacked enough easily accessible storage space despite the fact we organized every closet - houses just weren't built to be as convenient in the 1970's.
In the 8000 sq ft house we have
Second Floor:
5 bedrooms, 4 full baths, laundry room. One BR is used as my office, one is a guest room, one is master, one is for older child, and twins share a bedroom for the time being but can split if so wish in later years (and then there will be no guest space on this floor). 2 of the bathrooms other than MBA are doubles (shared jack & jill, and a large one in the twins' bedroom). Generous closets, we also have a huge closet above the 3-car garage which we use as part closet, part quiet sitting space.
First floor:
Living room/adults quiet conversation room (very necessary in an otherwise open floor plan with small children)
Dining room
Kitchen (very large)
Family room
Sun room - a very large space which currently serves as play room but once kids are older will convert nicely to an adult relax space
First floor in-law suite (for grandparents)
Mudroom with half bath, laundry area, planning space, closets, cubbies, extra fridge/freezer - this is very large and has many purposes, but with the kids, dogs, nanny, grandparents, garden we do need the workhorse space if we are to keep the rest of the house in reasonably good shape.
Basement:
Guest bedroom with full bath
Exercise room
Several storage areas
Rec room, with a generous size wet bar,
Game room space for pool table
Play area
For this many people, plus nanny coming in during the day it does not feel too large, but comfortable. Truth be told, if it weren't for the grandparents or the surprise twins, we wouldn't have done it, but we can afford it so no need to get on each other's nerves. The house looks very reasonably sized from the outside and people constantly comment on how deceptive it is once they get inside.
Once it is just the two of us (will that ever be, with kids returning home after college all too often?), it will be too large and we will look to downsize. I have started these conversations now (we are at least 15 years away from this) and I can see how it becomes very hard to consider smaller/older spaces once you have lived in these conditions.
We have friends who are a 3-person family in a 12,000 sq ft house. That does feel like a hangar, four years in many parts are still unfurnished. But they are in professions that require to "impress" in order to get business so that probably weighed in their decision.