I'm not sure how it works on IIs, but on linux-apache you can test for the total cumulative size by examing the content_length. I prefer to use this method, because it can stop the upload immediately. But I'm only an amateur at CGI.

if($ENV{CONTENT_LENGTH} > $maxsize){
 print "total files too large - must be less than $maxsize bytes";
 exit;
 }
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