I just finished a D9P quilt and it took 81 squares plus a 5" binding to make a 48" square quilt. So if you wanted it longer, you would have to add about 1 charm pack or 27 more squares.

Each 9 patch section would make a 13-1/2" finished square, so you need to figure out how big you want it, and then figure out the amount of charms. Remember, every charm pack has a different number of squares in it, some are more and some less than others. So, since I love using my lousy math skills for figuring out stuff, if you used 12 9 patch squares to begin with (before cutting it up), you would need 108 charm squares. This would give you a quilt top 40-1/2" x 54". Then you would need to add borders to make it bigger if you wanted it bigger. You can make it larger by putting sashing between the sets of 4 blocks, but I think that ruins the look of the D9P pattern. I've also seen someone make the D9P in a rectangle, then putting a narrow border around it in plain fabric, then a piano key border of charms, cut in half and sewn together in a random pattern for an outer border. That is in someone's album, and it is very sharp looking!