Presently the Rich Content Editor only offers Header 2, Header 3, and Header 4. I'd like to request that Header 1, Header 5, and Header 6 be added as options.
An explanation for the current choices I was given during an Instructure online support chat follows:
Header 5 is not an option as that would be the same size as regular text and function the same as bolding text. Every Canvas page already has a Header 1 built in (the page title gets that Header 1) and only a single Header 1 will be read so we don't give that option either.
My feeling is that the explanation does not hold up if an account admin applies customized CSS through the Theme Editor.
(Interestingly enough, the wysiwyg editor into which I'm typing this post offers all six header size options.)
I was surprised that a search didn't return similar requests for this improvement.