Good idea about proportional.
Just to be clear. There is quite a bit of control over the bounding box already. The only thing that you can't do is move its center and that is coming.
But today, you can do the following:
1. with proportional off, the sculptie takes the full bounding box
2. with proportional on, the sculptie takes the maximum bounding box that is possible while maintaining the shape of the sculptie.
3. with proportional on, you can set a multiplier less than 1 which will shrink the sculptie within the bounding box.