Short answer

Use ARRAY_CONSTRAIN(array,row_limit,column_limit)

Explanation

ARRAY_CONSTRAIN function could return the number of rows and columns that you want, just set the row_limit and column_limit parameters accordingly. To automatically set this parameters you could use a function like COUNT or COUNTA.

To calculate the number of rows/columns considering the data range by using built-in functions,

for rows use a formula like the following

=MAX(IF(LEN(A2:A)>0,ROW(A2:A)-1,0))

for columns use a formula like the following

=MAX(IF(LEN(1:1)>0,COLUMN(1:1)-1,0))

The sample formula in the question will look like the following:

=ARRAYFORMULA(
   ARRAY_CONSTRAIN(
     MMULT(N(ROW(A2:A)>=TRANSPOSE(ROW(A2:A))),SIGN(ROW(A2:A))),
     MAX(IF(LEN(A2:A)>0,ROW(A2:A)-1,0)),
     MAX(IF(LEN(1:1)>0,COLUMN(1:1)-1,0))
   )
)

Here is how I come up with a solution to my this problem. In case in future anybody gets stuck in something like this, hope this solve his/her problem and gets job done. :)

Google Script:

    function kalender() {
  var ss = SpreadsheetApp.getActiveSpreadsheet().getSheetByName("Form Responses 3");
  var lrow = ss.getMaxRows();
  var lcol = ss.getMaxColumns();
  var rng = ss.getRange(1, 1, lrow, lcol);
  var data = rng.getValues();

  for(var i=1; i< 100; i++){
    if(data[i][5] == 'new'){

      var subject = data[i][2] + " with " + data[i][1];
      var startdate = data[i][3];
      var enddate = data[i][4];
   //   var enddate = new Date(addDays(startdate,times));
      var event = CalendarApp.getDefaultCalendar();
      event.createEvent(subject, startdate, enddate);
      Logger.log('Event ID: ' + event.getId());

      data[i][5] = "added to calendar";
      rng.setValues(data);
    }
  }
}