For the best result, recommended background image dimensions are 2732 x 1536 pixels. This size fits different devices (iPhones, iPad, Android smartphones and tablets).
However, to make your game playable on different devices, you should fit game action, active objects and main design elements in the active area (2048 x 1536 pixels), as shown in the image below.
As you can see, you should make a wider background to fill wider screens of mobile devices.
You can download this scene template with all the file information here. It's a transparent PNG, so you can open it in any program and put this on top of your art to see what will be in the cutoff area.
If you want to make sure that all game objects are in the active area, simply play and test your game on TutoTOONS (normal screen size, not full screen).
New TutoTOONS design with canvas view shows background dimensions, no need to memorize them.
Now you can change the opacity of the scene background in Scene Settings. The two main uses: easy way to make the game background lighter and extra help in positioning game objects and images while building the game.
This feature is available only on TutoTOONS V3.