Hi..
By "Stage" I'm assuming you mean the working area rather than the Stage Prop that is included with CTA. Please correct me if I'm wrong.
The working area aspect ratio can be changed in the Export settings. From here you can change the Output Size values to be widescreen, square or narrow simply by entering the desired dimensions. As you do this you will see working area adjust to refect the changes.

When it comes to how your character or props appear in the working area you can simply use the controls on the top toolbar to zoom in so your character fills the entire working area.
Peter
Forum Administrator
www.reallusion.com