I believe you are looking for:
dbSetCameraView
This command will set the viewport of the camera. The viewport is the actual area on screen where all 3D is drawn. The default viewport area is the entire screen, but can be specified using this command.
Syntax
void dbSetCameraView ( float fLeft, float fTop, float fRight, float fBottom )
void dbSetCameraView ( int iCamera, float fLeft, float fTop, float fRight, float fBottom )
In which case I would also recommend looking up the "dbSetCameraAspect" function to make sure your viewport matches the correct aspect ratio.
In your DarkGDK installation folder is a really useful bit of documentation, open it up and look at the "camera" section for more information.
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