function Scanning_Gmail_Mailbox() {
  if (!UserProperties.getProperty('start')) {
    UserProperties.setProperty('start', '0');
  }

  var start = parseInt(UserProperties.getProperty('start'));
  var sheet = SpreadsheetApp.getActiveSheet();
  var row = getFirstRow();
  var ss = SpreadsheetApp.getActiveSpreadsheet();

  for (;;) {
    ss.toast('Now finding all the big emails in your Gmail mailbox. Please wait..', 'Scan Started', -1);

    // Find all Gmail messages that have attachments
    var threads = GmailApp.search('has:attachment larger:1m', start, 100);

    if (threads.length == 0) {
      ss.toast('Processed ' + start + ' messages.', 'Scanning Done', -1);
      return;
    }

    for (var i = 0; i < threads.length; i++) {
      var messages = threads[i].getMessages();
      UserProperties.setProperty('start', ++start);

      for (var m = 0; m < messages.length; m++) {
        var size = getMessageSize(messages[m].getAttachments());

        // If the total size of attachments is > 1 MB, log the messages
        // You can change this value as per requirement.

        if (size >= 1) {
          sheet.getRange(row, 1).setValue(Utilities.formatDate(messages[m].getDate(), 'GMT', 'yyyy-MM-dd'));
          sheet.getRange(row, 2).setValue(messages[m].getFrom());
          sheet.getRange(row, 3).setValue(messages[m].getSubject());
          sheet.getRange(row, 4).setValue(size);
          var id = 'https://mail.google.com/mail/u/0/#all/' + messages[m].getId();
          sheet.getRange(row, 5).setFormula('=hyperlink("' + id + '", "View")');
          row++;
        }
      }
    }
  }
}

// Compute the size of email attachments in MB

function getMessageSize(att) {
  var size = 0;
  for (var i = 0; i < att.length; i++) {
    //size += att[i].getBytes().length;
    size += att[i].getSize(); // Better and faster than getBytes()
  }
  // Wait for a second to avoid hitting the system limit
  Utilities.sleep(1000);
  return Math.round((size * 100) / (1024 * 1024)) / 100;
}

// Clear the content of the sheet

function Clear_Canvas() {
  UserProperties.setProperty('start', '0');
  var sheet = SpreadsheetApp.getActiveSheet();
  sheet.getRange(2, 1, sheet.getLastRow(), 5).clearContent();
  SpreadsheetApp.getActiveSpreadsheet().toast('Choose Scan Mailbox to continue..', 'Initialized', -1);
}

// Find the first empty row to start logging

function getFirstRow() {
  var sheet = SpreadsheetApp.getActiveSpreadsheet();
  var values = sheet.getRange('A:A').getValues();
  var c = 2;
  while (values[c][0] != '') {
    c++;
  }
  return c;
}

// Add a Gmail Menu to the spreadsheet

function onOpen() {
  var menu = [
    { name: 'Reset Canvas', functionName: 'Clear_Canvas' },
    { name: 'Scan Mailbox', functionName: 'Scanning_Gmail_Mailbox' },
  ];

  SpreadsheetApp.getActiveSpreadsheet().addMenu('Gmail', menu);
}