> When HTTRack produces a report like the one below, where
> does it get the file size information from? EG number of
> files written, bytes overall, bytes recieved, bytes/sec 
etc.
> Does HTTRack 'know' or calculate this itself

Yes, this is an internal computation. It is possible that 
not all files are being taken in account - I detected a 
related bug long time ago but couldn't reproduce it.
The bytes transfered should be exact, at least.