Although king size quilts are cited as 120" x 120", you may not want that much of a drop.
What I've done is take the number of blocks of the lap quilt (say, 16) and divide that INTO the number of blocks I'd need for the large quilt (say, 25). That'd be 25 divided by 16, which is 1.56. I'd multiply the fabric requirements for the blocks by 1.75 (1 bit more than 1.56 so I'd have extra blocks for pillow shams, a bed runner for a guest room bed, or just more blocks to choose from).