When I first started quilting (30+ years ago), I mostly did lap quilts (45x60). They were a good size for me to learn the basics, plus they weren't too bulky to quilt on my DSM. Then I went on to twin sizes as my kids grew up, then I did larger ones, when I found a local LA'er. 6 years ago, I got my own LA. Now, I mostly do K-Q sized quilts, but will do other sizes as needed for baby showers and so on.
I personally HATE square quilts (beds aren't square, are they?), and will often modify a pattern to change the square quilt to rectangular. And I really like my quilts extra long, to cover the pillows like a comforter. I've only done one "wall hanging" sized quilt - it's hanging on my quilt rack, next to some antique quilts I've found. Not hanging on the wall. Not really sure why, except maybe I can't figure out how to hang it up?!?