This is the exact length of the received HTTP body portion of the document. This does not count any HTTP headers that the web server may have sent prior to the HTTP body. If you edit a document in a text file and then load it into the web browser, this is the exact length of that text file in characters.
On the Macintosh operating system, some text editors hide additional resource data in the file. This is not included and on that platform, the fileSize property is a measurement of the data fork of the file and does not include the resource fork.