The tweeter displaces about 150 cubic inches. You can shrink the box that much if you want, it's about an eighth inch from each side. But the shift from 150 cubic inches is very small, and the direction is downward, which is better than upward.More important is that you shift the woofer up so that the distance between it and the tweeter is maintained. What I would suggest you do is to keep the box size the same, and use the additional room you'll gain by installing a brace from the front to back panels.