Thanks it was very helpfull. I've already found the solution. Here is my example
//gets the total number of users into all of the audiences of our campaign
public function getNumberOfUsersIntoAudiences(AdWordsUser $user) {
// Get the services, which loads the required classes.
$userListService = $user->GetService('AdwordsUserListService', ADWORDS_VERSION);
$conversionTrackerService =
$user->GetService('ConversionTrackerService', ADWORDS_VERSION);
// Create remarketing user list.
$userList = new BasicUserList();
//Create selector object which will be used during sending the get request
//to the server
$selector = new Selector();
//selector's fields property should contain a list of fields which we want to get from the server
$selector->fields = array('Size', 'SizeRange');
//sends the get request
$result = $userListService->get($selector);
return $result;