For me it often depends on the fabric. If it's a fabric I really really like and don't want to cut up, I'll do the whole quilt in nothing but simple 12" squares! I did that with the Tim Holtz fabric that came out recently; king size top, just 12" squares. I just need to get this puppy quilted and then it'll be my new bed quilt! (This one is MINE, MINE, MINE!) I also have a floral quilt in just 12" squares in progress, special request from my MIL; based on a quilt she saw in a catalog.
But then sometimes 2" squares are just the ticket! I make a pretty wide variety of styles so I don't have any one particular size that is my default. It's always changing.