Thanks a lot to Morton and Hidetoshi for their answers.

font.size and font.size = value are the most simple and logical to acess 
font size.
...
but as a Ruby newbie, I am not logical enough and I still need to get 
information from documentation and code.

Does it exist any documentation on Ruby/Tk ?
(it is not easy to get such information from the Tk code)

Thanks and regards,
Lionel