[O] change font-size in python plots depending on context

[johanna . may]

Dear org-mode fans,

I have various files that go together in one document, either
chapterwise or the whole book. And just as with tikz (latex) I want the font
size of python plots to change automatically when the context changes.

So I tried to define a variable in one of the header files like this:

#+name: fs
#+BEGIN_SRC emacs-lisp
10
#+END_SRC

I also tried to define it like this

#+name: fs
#+begin_src python :results output :session :exports none
fontsize=10
return fontsize
#+end_src

And then, in another file (that is being included by the header file) I
added :var fontsize=fs to the header and replaced the number by this
variable. I understand it complained that fs could not be found if I
used C-c C-c directly to test, since apparently the variable fs only
belongs to the other buffer (This is the first problem).

However, it doesn't work either when I compile everything (the second problem). 
Probably it
is also possible to solve this with some function in elisp. But, I have
to admit, that I haven't started learning elisp yet.

Does anyone have a solution to this problem? Any hints to helpful blogs
or stackexchange pages would be appreciated. I did a search but probably
not using the right keywords.

Thanks!

J