This workaround is pretty long.
Now we use
$bg = rwmb_meta( 'advantages_bg', array( 'size' => 'large' ) );
But the problem is when I need 3 sizes of the same image - then I need to call rwmb_meta 3 times, or use this workaround.
If the rwmb_meta returns sizes - why the size['file'] can't contain also directory? What is the intended way to use the size['file']?
I'd like to recommend to add "url" for sizes.