Upload file size limit

[mybox]

ragazzi ho questo codice:

if($action=="uploader")
{
addvisitor();
addonline(getuid_sid($sid),"User vid Uploader","");
$nick = getnick_sid($sid);
$uid = getuid_sid($sid);
echo "Welcome $nick
";
//get file name
if ($upload="upload"&&$file_name){
if (!eregi("\.(3gp)$",$file_name)){
print "[b]Unsuported File extention!!![/b]";
}else{
$file_name = preg_replace(
             '/[^a-zA-Z0-9\.\$\%\'\`\-\@\{\}\~\!\#\(\)\&\_\^]/'
             ,'',str_replace(array(' ','%20'),array('_','_'),$file_name));
if(strlen($file_name)>53){ print "[b]File Name to long!!![/b]";
}else{
if (empty($file)) {
print "[b]No input file specified!!![/b]";
}else{
if (file_exists($file_name))
            {
            echo "Pic already exists";
            echo "
[size="1"]<a href=\"upload.php\">Upload xhtml</a>
[/size]";
                  echo "
[size="1"]<a href=\"index.php\">Uploads</a>
[/size]";
                  print "</p></body></html>";
            exit();
            }

$pics = mysql_fetch_array(mysql_query("SELECT COUNT(*) FROM vidoes where uid='".$uid."'"));
$cpic = $pics[0]+1;
$rev = strrev ($file_name);
$exp = explode (".", $rev);
$true = strrev ($exp[0]);
$trueext = strtolower ($true);
$picn = "$nick$cpic.$trueext";
copy("$file", "vidoes/$file_name") or
die("Couldn't copy file.");
$adds = mysql_query("INSERT INTO vidoes SET uid='".$uid."',file='$file_name',fname='$fname'");
    if ($adds) {

          echo "added pic $file_name
";

         } else {

              echo "Please try again
";
           }


echo "file has been successfully uploaded";
}
}
}
}
?>
<?php
echo "<form align=\"left\" action=\"uploadvid.php?action=uploader&amp;sid=$sid\" method=\"post\" ENCTYPE=\"multipart/form-data\">
File Name:
<input type=\"text\" name=\"fname\" size=\"30\" />

File: 
<input type=\"file\" name=\"file\" size=\"30\"/>

<input type=\"submit\" value=\"Upload!\"/>

[size="1"]<a href=\"index.php?action=main&amp;sid=$sid\">Home</a>
[/size]
}
echo "</form>
</body>
</html>";
?>

a questo script vorrei aggiungere un file size limit di 300kb. qualcuno riesci a definirmi tale limite?

grazie mille