Note: After saving, you have to bypass your browser's cache to see the changes. Google Chrome, Firefox, Microsoft Edge and Safari: Hold down the ⇧ Shift key and click the Reload toolbar button. For details and instructions about other browsers, see Wikipedia:Bypass your cache.
/** Do not install this directly. See https://en.wikipedia.org/wiki/Wikipedia:Prosesize. */
mw.loader.using( 'user.options' ).then( function() {
	if ( mw.user.options.get( 'gadget-Prosesize' ) === null ) {
		new mw.Api().saveOption( 'gadget-Prosesize', '1' );
		mw.loader.load( [ 'ext.gadget.Prosesize' ] );
	}
} );