Note that this is not fool-proof, but it should work on most servers and urls that reference static files.

If you have the lwp command line tools installed, you can do:

> lwp-request -m HEAD http://example.com
[download]

LWP::Simple has an head() function, which returns the content-length as the second return value.

For this task I wrote HTTP::Size.

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];

   The Content-Length entity-header field indicates the size of the
   entity-body, in decimal number of OCTETs, sent to the recipient or,
   in the case of the HEAD method, the size of the entity-body that
   would have been sent had the request been a GET. [...]

   Applications SHOULD use this field to indicate the transfer-length 
+of
   the message-body, unless this is prohibited by the rules in section
   4.4. [...]

   Note that the meaning of this field is significantly different from
   the corresponding definition in MIME, where it is an optional field
   used within the "message/external-body" content-type. In HTTP, it
   SHOULD be sent whenever the message's length can be determined prio
+r
   to being transferred, unless this is prohibited by the rules in
   section 4.4.
[download]