There are two answers to the question. Or, more accurately, there are two questions.
One question is "What is the smallest space we can cram into?" The answer to that is--for now--one of the studios with the trundle beds. When you have six over three, the answer is a 2BR. The other question is: "What size unit would make for a reasonably enjoyable vacation?" The answer to that?
Your Mileage May Vary.
I was just saying this in another thread, but the entire reason we got into timesharing was to avoid having to cram the four of us into a hotel room. Our first
DVC stay was in a 1BR, and we quickly decided that we would much rather have a 2BR, even though we didn't "need" one. Having a dedicated space in which no one has to sleep is great for early birds, night owls, and afternoon naps.
Right now, I go by the following rules of thumb, at Disney or elsewhere: One person can do a studio, but a 1BR is better. Two people? Strong preference for a 1BR; it is almost but not quite a requirement. Three people? We can squeeze in a 1BR if we have to, but would strongly prefer 2BR. All four of us? 2BRs are the minimum, but I will sometimes splurge on a 3BR. After all, the kids had separate bedrooms when they were at home, and while they each have a housemate, they each also have their own private bedroom. Why not do that on vacation too?
So far, I've never put my partner and I in a studio. I've booked a couple for us, but ended up cancelling them in favor of something larger. I had a "studio" for myself exactly once, but it was a loft with a king bed upstairs and a fully-equipped kitchen with apartment-sized appliances, so it hardly counted. (It also had a resident tattoo artist, and was, ahem, "colorful." Long story.)
When the kids were younger, my partner and I would often find ourselves hanging out in the living room of our timeshare condo, while the kids were off in their bedroom(s). One of us would look at the other and say: "Hotel rooms, huh?" As in: we understand that families like ours often vacation exclusively in hotel rooms, but we have no idea how they do it without divorce or murder.
(PS: the other cool thing about the villas? The master bedroom has a door that locks, meaning the parents cannot be interrupted
in flagrante delicto. After all, it is your vacation too.
)