wrap content with column and add widget like container set that width

showDialog( context: context, builder: (context) { return AlertDialog( title: const Text("Bookmark"), insetPadding: const EdgeInsets.all(0), shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(10)), content: Column( mainAxisSize: MainAxisSize.min, children: [ Container( color: Colors.red, width: screen.width * 0.70, ) ], ), ); });

just wrap content with column
showDialog( context: context, builder: (context) { return AlertDialog( title: const Text("Bookmark"), insetPadding: const EdgeInsets.all(0), shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(10)), content: Column( mainAxisSize: MainAxisSize.min, children: [ Container( color: Colors.red, width: screen.width * 0.70, ) ], ), ); });