The request to add some more text formatting buttons to the edit box has been made before, but currently, apart from the buttons to add headings (which do affect font size) and bold and italic effects, there is no way to adjust the presentation of your text without turning to HTML and CSS code.
Assuming that you're not expecting all your users to fill your forum with a glaringly inappropriate mish mash of gaudy writings and such posts will be limited to a select few authors for special announcements only, then the best thing to do would be to get them to use a simple WYSIWYG web editor software package that could generate the code and paste that into the forum, with a mark in the "Message is in HTML format" box.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.