How does Firefox(or dld manager) determine the size of a file being downloaded?

Actually what I want to do is to FOOL the brow开发者_如何学Goser by saying that the file to be downloaded is 1KB, no matter what the file size is but it should download the entire file!

Content-length: 1024 in the headers, but some browsers make take that as gospel and cut off the transfer after that kilobyte's transferred. You have no control over what the client does with a download. At most you can give some hints (content type, length, disposition) and after that it's out of your hands.