In top view or side view, resizing is free, but in camera view resizing is ignored.
Is this what it is? Or are there any options I’m missing?
And I took the GIF to ask this question, but I do not know how to apply the HTML code since the site form changed… How do you attach images?
Attach link instead of HTML.
changing the size frome camera view is not possible.
If you Change the axis for this background image to camera, you’ll see that the size option disappears.
Unfortunately I cant tell you the reason why this happens. Maybe because you’ll normally want the image to fit the camera size, I dont know.
Maybe add this image as a plane and rotate it towards the camera?
I’m sorry about the limitation of the background image view function.
But you are right.
I tested it as you said, and I got results similar to the background image view by making the plane a child of the camera and adjusting the texture transparency.
Thanks 
And I realized that the layout images usually has more space than the camera views.
I think that the sources for compositing has the perfect size for 1920x1080 is a bad idea that did not consider the modification in the future.