@felgallAug 09.2007 — #JavaScript has no concept of a file at all so the only solutions to this are to use a proprietary addon that is browser specific. Since Firefox attempts to follow the standards and does not have proprietary codes built in the only option would be to attempt to find an extension that provides that functionality.
@Eddy999999Aug 09.2007 — #If it's a server-side file, you could use AJAX to access a PHP function which gives you the size of file. If the file is on the client-side though, you're pretty much out of luck, I think.