CST bounding box size manipulation

In CST I know that the simulation of the structure is performed within the bounding box of the structure. But size of the bounding box is not in my hand. For example. I created the 5x5x5 cm vaccum box in which I want to see how Gaussian excitation source will create the E field inside that vaccum box and how that E field will propagate with respect to time and space. But I gor the bounding box bigger than that structure. Is there a way to modify the size of bounding box?

Go to boundaries and set all boundaries to "open", without adding space. Now You can control the domain size by the size of Your vaccum box.
Regards