I try to think of the recipient when I decide on shape and size. My baby quilts are mostly square. Military - slim twin to fit on a bunk. Wedding - usually a large rectangle to use as a couch throw, but big enough for two. The only quilt I'm making for me is a rectangular Double Irish Chain. If I ever make one for my bed, it will be square for my queen size bed with a thick mattress.
I have a king size being FMQ - it's not going well with my little machine. It's a wedding quilt for my son. I have one being pieced (another wedding) that is growing. I think I will downsize it while I can.