j_mart

В обработчике события 'part' (part) эта информация недоступна.
Она доступна или в обработчике 'file' (name, file)

form.on('file', function(name, file) {
  console.log('File name: ' + name);
  console.log('File size: ' + file.size);
});
/*
name - the field name for this file
file - an object with these properties:
  fieldName - same as name - the field name for this file
  originalFilename - the filename that the user reports for the file
  path - the absolute path of the uploaded file on disk
  headers - the HTTP headers that were sent along with this file
  size - size of the file in bytes 
*/

или в дефолтном form.parse(request, [cb])

form.parse(req, function (err, fields, files) {
    for (var name in files) {
        console.log('got file named ' + name + ' with size = ' + files[name]['size']);
    }

    console.log('Upload completed!');
    res.setHeader('text/plain');
    res.end('Received ' + files.length + ' files');
});