MOST of my quilts are square! Mine are usually right around 72x72, somehow that's the size I settled on and all my patterns seem to work out well in that size. I call them "napping quilts" or "oversized throws" and I love them. It's big enough that if we're sitting up to watch a movie it covers both of us. If I'm napping, it's just the right size to wrap up in and be covered from chin to toes. They fold up into squares that fit perfectly on the end of the couch.
I've made very very few bed quilts since I make my quilts as gifts, and without knowing how people make their beds or if they have foot boards, etc. it's tricky to know how big or how small to make the quilt and where to place the focal point. I AM making two bed-quilts this year for Christmas gifts; one is for my grandmother-in-law who is in a hospital bed, so hers will be specifically sized for that; since she's always in the bed I don't need to worry about how it gets made up. The other is for my MIL; she has this adorable antique bed she sleeps in and it's only a double; I was able to sneak some quick measurements against my arms for how much of a drop she was using and see how she makes her bed so I think I will be able make hers the right size or pretty close to it; but hers is going to just be big floral blocks so there's not a big central medallion I need to worry about centering properly on that one.