Allow users to specify the size of the file on the remote HTTPS server

Created by: teor2345

Following on from #123 (closed), I suggest we allow the following option:

file_size: The size of the remote file, in bytes. sbws can measure relay speeds up to file_size / target_seconds. Faster relays will all have this maximum speed. (TODO: reword?) If not specified, defaults to 1 GB. (optional)

Then we don't have to upgrade the protocol to change the file size.