I'm not sure if this can be done when requesting user favorites, as I've never used that feature, but when requesting videos via search terms you can set the results number via the setMaxResults method. You might be able to work this in with your user favorites request.
Here's a snippet of code we use:
$yt = new Zend_Gdata_YouTube();
$yt->setMajorProtocolVersion(2);
$query = $yt->newVideoQuery();
$query->setOrderBy('viewCount');
$query->setSafeSearch('strict');
$query->setFormat($searchFormat);
$query->setVideoQuery($searchTerms);
$query->setMaxResults($limit); // number of returned results set here
$query->setStartIndex($offset);
$results = $yt->getVideoFeed($query->getQueryUrl(2));