20.48 is called a Bryce unity. All objects are created at 20.48 cubed bryce units except terrains which are created at 4 time the x/z dimensions. This is simply the default size setting for Bryce so that when you select "unscale" in one of the menus on the edit pallette the object will snap back to this default size. You can change the unity setting for this session of Bryce( when you exit Bryce it gets reset) by changing your object, go to attributes, click the little triangle next to absolute coordinates and select set as unity. Now when you unscale the object will snap back to your modified size and shape. There is a whole chapter in "Real World Bryce" on the coordinate system, chapter 6 to be exact. What reason would you want to change it (actually there's no way to change it, all objects created are at this size)? The halving explained above is simply hitting the divide key on the keypad (/), you can double your object by hitting the multiply key (*).