Hi Chris,
With Javascript and jQuery, sky is the limit. What you seem to be looking for is some custom container that will have that embedded. I haven't seem anything like that yet.
But here is what I found that might be able to help you out doing that: Allow users to change font size in a webpage
Of course those concepts would have to be adjusted accordingly to only apply inside the container, but that is where further jQuer and Javascript would be required to make this work as you asked.
Cheers,
Aderson