Presuming the server you're downloading from is sending the proper headers, it should send a Content-length: header. From the LWP::Simple POD:

head($url)

    Get document headers. Returns the following 5 values if successful
+:
    ($content_type, $document_length, $modified_time, $expires, $serve
+r)

    Returns an empty list if it fails. In scalar context returns TRUE 
+if successful.
[download]

Therefore to make a determination based on filesize:

$content_length = (LWP::Simple::head($url))[1];