When I make what I call a "snuggle quilt" - my definition of one that would be used when lounging around watching tv or curled up on a chair reading - would be wide enough to tuck in around the sides and long enough to tuck under your feet at the bottom and come right up under your chin with some "wriggle" room would of course depend on the size of the person who it is being made for.
In other words I "custom" size those quilts rather than just picking a standard bed size.