In /portal_tinymce/@@tinymce-controlpanel - Styles add definitions:
Font size 8|span|custom-font-size-8
Font size 9|span|custom-font-size-9
Font size 10|span|custom-font-size-10
etc.
Meaning you can select the options Font size x in your editor. This will be saved as a span with class custom-font-size-x.
When you save this it will already work. But you need the styles to make this visible.
I had an override to tinymce editor already set so I added the styles for each class like: