Thanks Dale. Based on that example, I can set the fontsize
> from a script. Is there a way to do it from the rc file?
> It would be great to set it once with the rest of my font
> preferences.
No, but it would not be difficult to add these parameters in rc.
Ready to become an mpl developer? 
Look at matplotlib/__init__.py to see how the rc params are
processed. Add the ones you want and use them in legend.py, following
the example of Text or Line2D in matplotlib.text and matplotlib.line2d
respectively.
JDH