In DNN 9x the default font sizes are not very helpful. Here are some that are more useful

12px;14px;16px;18px;20px;22px;24px;26px;28px;30px;32px;34px;36px;40px;
44px;48px;1rem;1.25rem;1.5rem;1.75rem;2rem

The setting to put this into is described here:
https://dnncommunity.org/blogs/Post/8202/Customizing-DNN-s-Default-HTML-Editor-Provider-CKEditorProvider