If I have to fit a quilt to a bed, then I make the quilt PLENTY big enough.
Quilts get a bit smaller in the quilting process. Cotton shrinks when it is washed.And I want a bed quilt to not only fit over the bed, but to fit well over my husband and me when we are both in the bed under the quilt, with no tug of war over the quilt.
If I am just making a quilt to make one, I like to make sofa size throws, about 50 x 70 inches. That is small enough to finish, but big enough to show the design repeat well. It is also usuable for Project Linus or for Quilts of Valor if I decide to give it away.