I love the idea of living in a tiny space, but practical reasons unique to us get in our way. Our jobs are in the suburbs, where there aren't a lot of studios or small spaces that are close to work for us (I prioritize a short or non-existent commute for my husband over a small space). My husband has pretty severe allergies to pollen and pets, so we have to be in a space that has not had pets in it (their dander gets all up in the heating vents and then it's pretty much there forever), and it well insulated/filtered from the outdoors (so I don't know that RV living would actually work for us, as much as I wish it would!).
And since we had a baby, I'm a stickler for having more than one toilet. When I was a kid, I was in a house with 5 people (2 of them teenagers, even) and one bathroom... I don't want to do that again. All it takes is for everyone to get food poisoning for that to become a really messy situation (anyone remember that scene from Bridesmaids?).
All of that said, we're in a condo that's about 950 sq ft now (and conforms to all of the above needs), and we're pretty happy with this much space for 2 adults and an infant. I can't imagine we need more, though a dedicated home office space would be nice since I work from home currently, it's not really necessary. We moved my desktop computer into the master bedroom so that I can close the door to the rest of the apartment, and that works fine, even if it looks a little "college student-y."