If I was making what I call a "couch quilt" - big enough to cover a tall adult sleeping on a couch - I would make it approximately 60 inches wide - and 84 to 90 inches long.
I like to be covered up to my ears.
How about taking some yardage and just go lay down and cover yourself up and see what you think?
If you mean something just to cover someone's legs - approximately 45 x 60 would probably be adequate.
I know what you mean about opening up one's mouth - I was offering to make my DIL a quilt - and what I had in mind was a couch quilt but I didn't specifically say that - and she wants a KING size quilt. Man, those beds are BIG. I feel very overwhelmed by that project.