The size of the state space window can be set by selecting the menu option Options->Build State Space->Set Step Window. This will open the dialog displayed below.
Type in the desired size of the window in the text field and press the OK button. The new window size will after that be used when computing state spaces with the Bounds Editor.
The window size is the approximate maximum of states in the temporary state space that is used in forming the result state space. The smaller the number the smaller the amount of memory the computation will require. The drawback is that using a smaller window size will slow down the computation and if the window is small enough the computed state space may not be complete.