I know with my bed I made one 85 inches across and it JUST makes it. When I did the moose and train quilt I made it 88 wide and they said it fit perfectly on their bed. Now a friend of mine has the super deep mattress with the pillow top on it. That one I would have to make it 90 in order for it to cover the sides of mattresses.
I got a chart one time from a store (can't remember which) that just gave the top measurements of the sizes. The drop was not included.
I like the longer quilts anyway so when I make more for my bed.........the bigger the better!!
Barb