MOST of my quilts are all the same size, I call it "nap quilt" or "couch quilt" size - 72" x 72"
Why that size? Because it's the size *I* like best for myself! It's long enough to cover me from my feet up to my shoulders, and wide enough to wrap around me plus a couple of small dogs when I'm reading or napping on the couch. Or if DH and I are watching a movie, it's long enough to cover our feet and our laps and wide enough for us to share it. (Again, usually with a dog or two in there somewhere, and maybe a cat.)
Plus I really like square quilts for some reason. I do make bed quilts now and then, but the big 72" square is my favorite.