Expose flag_size

If you take [this example](https://github.com/MartinThoma/algorithms/blob/master/PDF/PDF-export-example.pdf), then you need to set two flags to get a decent output:

* flavor="stream"
* flag_size=True

The first one seems to be handled automatically, but the second one has a major mistake. Would it be ok to add a checkbox for that? (I can also do it, but I want to know first if this is desired)

Start with the handling of the existing flavor option and the flag_size setting, then compare the interface with the linked PDF example. Add a checkbox that exposes flag_size and verify that selecting it produces the expected table output for that example.

Written by the indexing model from the issue text.