I want all of my quilts to hang 15-18" on each side, so minimum 15" on all 4 sides, and that 15" can be used for the fold under and back over the pillow. I would agree with others here that 12" is needed for the fold under the pillow. If I remember right, a queen sized bed is 72x80, so I would want something around 102 x 110".
For reference, a king sized batting is 120x120, and a king bed is 80x80, which probably gives you an 18" overhang if you don't use quite all the batting.