Chris -
Take a look at this url -
http://php.chek.com/manual/features.file-upload.php
Other than that, you're pretty much stuck - the server can't evaluate the size of your file until it is actually ON the server. at which point, you could go ahead and trash the file, returning an error page to the user.
in your php.ini file there is also a max set upload_max_filesize, I believe, which you can change with the proper code.
I honestly don't remember if I was ever able to do this, though I believe I tried at one time.
-S