Change Font Size in Post Dynamically

I looked around and couldn’t seem to find a definitive answer to this one. I have a lot of users that aren’t particularly web savvy. Is there any way natively or via plugin that I can set it up so my users can change their font size on a post-by-post basis. Most of them are used to free blogging software (e.g. blogger, blogspot) and would like to simply highlight text and select a new font size for the blocked text.

Thank you in advance!