Согласен с @ Jon
$ch = curl_init();
curl_setopt($ch, CURLOPT_HEADER, true);
curl_setopt($ch, CURLOPT_NOBODY, true);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);
curl_setopt($ch, CURLOPT_URL, $url); //specify the url
curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);
$head = curl_exec($ch);
$size = curl_getinfo($ch,CURLINFO_CONTENT_LENGTH_DOWNLOAD);
if(<limit the $size>){
file_get_contents($url);
}