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);
}