I want to change the font-size in the editor, but it seems like modifying editor-style.css cannot make any difference.

Do I miss anything?

Thanks.

Are you talking about the wordpress back end editor?

Yes. The editor in wordpress post/page edit page.

that' a question you should post over at wordpress.org

The style of the editor can be customized by css, more specifically editor-style.css, which is a part of the theme.

OK. I just figured it out.

line 912 in functions.php:

add_editor_style();

it is commented. Just bring it up and make it before the line 911 "/*".