[Admin]

Ok, first of all make your attachment limit higher, let's say 5MB. Now edit functions.php and replace this line:

    $visible = iif($moderate,0,1);

with this:

    $visible = iif($moderate or $filesize>1048576,0,1);

1048576 = 1MB. You can change it if you want.
Now every attachment bigger than 1MB will go into moderation, and attachments bigger than 5MB won't be accepted.