Custom image size in Media Dropdown

The above problem got fixed after regenerating the thumbnails and by using the following function.

function mytheme_image_size_names( $sizes ) {
    $sizes['blog_body_img'] = __( 'Blog Body Image', 'isaumya' );

    return $sizes;
}
add_filter( 'image_size_names_choose', 'mytheme_image_size_names', 11, 1 );