If you want have a long text to display in the context, you need to set a custom content.
All you need to do, is to add a style to your content like below:
  private void generateNotification(Context context) {
        String message = "This is a custom notification with a very very very very very very very very very very long text";
        Bitmap largeIcon = BitmapFactory.decodeResource(getResources(), android.R.drawable.ic_dialog_alert);
        NotificationCompat.Builder builder = new NotificationCompat.Builder(context);
        builder.setContentTitle("Title").setContentText(message)
                .setSmallIcon(android.R.drawable.ic_dialog_alert)
                .setLargeIcon(largeIcon)
                .setAutoCancel(true)
                .setWhen(System.currentTimeMillis())
                .setStyle(new NotificationCompat.BigTextStyle().bigText(message));
        Notification notification = builder.build();
        NotificationManagerCompat notificationManager = NotificationManagerCompat.from(context);
        notificationManager.notify(101, notification);
    }
 
                

