showModalBottomSheet(
  backgroundColor: Colors.transparent,
  context: context,
  isScrollControlled: true,
  isDismissible: true,
  builder: (BuildContext context) {
    return DraggableScrollableSheet(
      initialChildSize: 0.75, //set this as you want
      maxChildSize: 0.75, //set this as you want
      minChildSize: 0.75, //set this as you want
      expand: true,
      builder: (context, scrollController) {
        return Container(...); //whatever you're returning, does not have to be a Container
      }
    );
  }
)