I think 160 isn't right, since that's not divisible into 9...
So, let's say you used 162, since that does divide into 9.
If you did a normal 9-patch block (pretend no cutting yet!), you would get 18 9-patch blocks, right? And each 9-patch would be around 14". So then laying them out 3x6, the quilt is 42" x 84" (a weird layout, sure, but the best I would do with 18 9patches). So other then needing an extra row, I think that's pretty good.
I would suggest making 24-25 normal 9-patch blocks (gives you a final arrangement of 4x6 9-patches or 5x5 = 216-225 charm squares). Before they get cut, that gives you a quilt size of around 56" x 84" or 70"x70". After the 9-patches are cut and reassembled, you'll get in the range of 53" x 81" or 67"x67" (rough math), both of which are reasonable sizes.
So if you work in the range of 24ish regular 9patch blocks, you'll get a decent quilt size. I think what might help you is to reassemble the blocks into new 9patches... might help you wrap your brain around the size easier :D