I say this as a person who has never made one but I would want mine a little longer so that I could cover my toes. My feet are always cold and a quilt that only comes down to my ankles while sitting wouldn't get used very much. I want enough to tuck my toes under. Some people only have socks on when sitting in wheel chairs and I always think their toes must be chilly. I would say it would be more of a rectangle than square and I think flannel would be perfect--again I am always on cold side so I think everyone is cold :-)
Can't wait to see your projects
Tammy