Get the total size of a folder in Google Drive

folder-sizer.gs

// adapted from https://webapps.stackexchange.com/a/123681

function doCount() {

  var FOLDER_ID = "<folder_id>";

  Logger.log("Getting folder sizes...");
  var totalSize = countFolder(DriveApp.getFolderById(FOLDER_ID), "/");
  Logger.log("Folder Sizes Report completed");
  Logger.log(Utilities.formatString("TOTAL SIZE: %12.2f MB", (totalSize / (1024 * 1024))));
}

function countFolder(folder, path) {
  var totalSize = countFilesIn(folder);
  var children = folder.getFolders();
  while (children.hasNext()) {
    totalSize += countFolder(children.next());
  }
  return totalSize;
}

function countFilesIn(folder) {
  Logger.log("Counting files in " + folder);
  var filesSize = 0;
  var files = folder.getFiles();
  while (files.hasNext()) {
    filesSize += files.next().getSize();
  }
  return filesSize;
}