add support for SIZE (only manually tested)

I read the note on unit tests but:

1. need a mock FTP server, not that trivial?
2. there's no current testing framework in place (e.g. which one should be used, the Python built-in one will probably suffice but pytest is more readable / less boilerplate .. etc.)

Anyhow, would completely understand if you reject the pull req :). It was written quite quickly. Seems to be a pattern with programmatic FTP usage. I basically just wanted to point the code out - if I have more time one of these weeks I ought to make a more serious attempt at a contribution..