thanks for fast the answer!
Anyway, can I be sure to don’t loose important CSS rules doing in this way?
I mean, this tutorial says to import all CSS template files:
http://www.joomlacontenteditor.net/support/tutorials/editor/item/create-a-custom-editor-stylesheet?category_id=53
(anyway I don’t understand why the author suggest to import all CSS files listed in the front view code, while we are talking about the back end view)
I’ve tried to import all CSS files listed in the front view code; and yes, the font size can be changed, but some things don’t show really well (some elements seems to be missed, as list styles); apparently seems to work fine without importing nothing, and just putting font-size rules for the body in the new editor.css files.