Hi,
that's quite strange. Please try this.
1. Open Cheetah3D.
2. Resize the window to full screen size.
3. Close the window by clicking on the close window button.
4. Quit Cheetah3D.
If you now start Cheetah3D the window size should be the full screen size.
Bye,
Martin
P.S. Why is your menu bar so short?