Right now for me it's resale value. I need to be able to sell this place when I retire. It needs to be big enough for that.
For my retirement place I need a modest bedroom, guest room, office, kitchen, dining room, laundry/2nd bath, and a medium size living room. Plus shop. Enough room for me and a couple of guests.
If I had family living with me I'd need enough space for family members to have privacy, especially me. The total space would depend on how much family.