I see you changed the scope of your question with your edit, but I have a solution that will change the content editor default.
This will change the default font size in the wiki editor:
.ms-wikicontent, .ms-wikicontent p,
Just add it to your master page or to your custom css somewhere. Or you could even put it in a content editor for a particular page.
The declarations cover not only regular wiki content, but wiki content whilst in edit mode - both for the
p element and for text without the
p element. Apparently, the style class changes when you enter edit mode.
I want to rather select (as default) the second (or another) option
from font-size drop down menu than change all with CSS.