Use Crl+Arrows keys to move the bounding box in the position required.
Use X,Y,Z keys to increase the bounding in the corresponding dimension and Shift-X, Shift-XY Shift-Z to decrease it.
Switch between wireframe and shading modes to get a better view.
Press "Enter" to execute the operation or Esc to abandon it.
The results of the operation are presented in the new 3D-Viewer window.