I'd recommend running the game and stage live in play mode, pausing it like you show above, then positioning the background to the size and position you want. You might have to move it along the z-axis as well. Then end Play Mode, open the stage prefab or scene, and apply the modified transform.

The issues if you're using a skybox is typically because the Skybox is normally designed to be rendered at 60 FOV but UFE runs around 15-20 FOV by default. So you'd want the default camera to ignore Skybox rendering, and then make a sub camera dedicated to rendering exclusively the Skybox at 60 FOV.

Also, if using 2D backgrounds for stages it's normally best to make sure your Global Config > Camera Options > Enable LookAt option to be disabled. This option will rotate the camera on the y-axis as players move on the stage when it's enabled. When It's disabled it will do a panning/transform along the x-axis.