Good question. At face value 12 inch drivers will put out a bigger soundstage than 3 inch drivers. I find it depends on the listening room. Large tower speakers with multiple 8+ inch drivers will put out a large soundstage if properly designed and built but can over power a smaller room. By the same token a pair of bookshelves with 6 inch drivers that are a great match for a smaller room can be limited in a larger room. I have both types of speakers in two different rooms and matched well they each sound excellent with a soundstage that fits the room.
There is no substitute for cubic inch or in this case surface area
After listening to quite a few speakers, my conclusion is that if you want large enveloping soundstage, you need a lot of drivers.
I once had a speaker with two 12in. drivers and the soundstage is just floating in the air. None of my other speakers could do that.
Currently I have a pair of Thiel CS2.4. It is a very good speaker but with small drivers there is really limitation to what it can do in term of soundstage size. I really miss that.