Add this to child or parent theme. I think either way works.

functions.php:
add_theme_support( 'editor-styles');
function transfiguration_theme_add_editor_styles() {
    add_editor_style( 'style-editor-transfiguration.css' );
}
add_action( 'admin_init', 'transfiguration_theme_add_editor_styles' );


style-editor-transfiguration.css  (new file somewhere in the same theme folder as the functions.php file)
.wp-block {
    width: 100% !important;
    max-width: 100%;
}