Check this 
:
CameraSize.zip (2.7 KB)
1- In the Layer Editor, select your layer, then select the camera, and set the camera size.
or 2- Use the action: “Layers and cameras ==> Modify the size of a camera”
Note: If the game’s window and the camera (through the camera’s render zone) haven’t the same proportion, the render will be stretched, as in the example 