Interesting topic, I both quilt and cook large. Find it hard to make just one reasonable portion...
My natural design sense/inclination is for rectangular queen sized bed projects. Even for twin sized I still want about 90" in length. It is almost impossible for me to design within baby sizes, mine are definitely more crib than travel sized. And then although I did it once as a challenge, it is almost impossible for me to design within preemie sizes.
For couch or comfort size, again I still prefer rectangular but am more open to square. I think I'm going for a minimum of 6 ft (72 inches), think a wingspan or how far out your arms can reach, with about 4 ft (48 inches) in depth.