If you want to set the image size to full-width for use in the post editor permanently.
function mcw_imagesize_setup() {
$image_set = get_option( 'image_default_size' );
if ($image_set !== 'full') {
update_option( 'image_default_size', 'full' );
}
}
add_action('admin_init', 'mcw_imagesize_setup', 10);