Actually, it does work. At least it does for 1.9.3. Keep in mind your browser caches css files, so you might have to force it to refresh (Ctrl-F5) after making a change to the css. Also keep in mind if you don't edit the correct css file for the correct theme that you are using, you won't see the change.
Also keep in mind the post editor you are using to write your posts might not have been written by the YAF programmers, so changing the behavior within the post editor you are using (for example, to change the "default" from 5 to 6) might be out of scope for this forum's community but rather the authors of the specific post editor. YAF provides the ability to work with many editors, so maybe you should switch to one that provides the functionality you are seeking.
In this post, someone asked how to easily change the displayed font sizes, and my answer stands. At least, for the current version (1.9.3).
If you can't get it to work, provide some more details as to what version you're using, what theme you're using, and a link to your forum so we can see that you actually made the change correctly.