Example: top mattress is 60 inches wide, 80 inchesong, a d 12 inches thick.
Assume there will be some "shrinkage" from the quilting . There will probably be additional shrinkage if one uses unwashed fabrics and batting that will shrink when it is washed.
I prefer to not have the quilt go over the pillows.
So - I use preshrunk fabrics and batting - so I only have to consider how much the quilt will "shrink" from the quilting.
Minimum size for me for the top would be about 88 inches for the width (12 + 60 + 12 + 4 inches for shrinkage)
and about 97 inches for the length (12 + 80 +5 inches for shrinkage) - assuming approximately 5% shrinkage.