You did misunderstand, or I was unclear. First and foremost the size of a file is not the same as the size of it's contents (especially on some filesystems). But for all but the strictest of purposes it's close enough. What I suggested was something like:

#About to append $message, do we have enough room?
if( -s $file + length($message) > $maxSize ){
  #do magic here
}
[download]

--
perl -pe "s/\b;([mnst])/'\1/mg"