Ok I actually wanted to try passing -x and -y but it looks like libtmux new_session already does that with specific large values if the tmux version supports the arguments, so what I specifically wanted to do seems to be covered.

Ok, I think this is covered by the options setting? I just didn't notice it in the documentation.
It would be nice to have an explicit list of options separate from the mass of examples (which is indeed good to have though).