There are times where the web server can't determine the length of the content, such as with a script. Additionally, there are times when you would think the file being returned is static enough (like a download of an MP3), but in reality there is a script behind the scenes that's actually bouncing the contents to you (such as using sessions ID's or such) such that the server itself never knows EXACTLY what the file is, and therefore can't
I hate to give answers like this, but: tough. Even download managers (like GoZilla and Download Accelerator) can't always determine the filesize.