Mike, I found this old bug on the CKEditor developer site:
They closed the ticket as "wont' fix". So, I think that means you have to manually edit the HTML source. In this case, I've manually edited the HTML here to set the <ol> tag's font size to 20px.
EDIT: On my first post, the text in the list was all 20px. After an edit, I cannot make it work again. I think CKEditor is stripping my font-size styling.
If I recall correctly, there's an option in the widget to enable a "view source" button, where you can make this edit manually.
The only other solution I can think of would be to wrap your output HTML form the CKEditor in a div with styling applied that sets the font size. Hopefully then that styling would get inherited by the rest of the document, including the numbered list.