There’s only two ways that I know of which one is using a plugin, the other is that browsers of today have the ability to resize text in a page of any web site. For plugins, have you tried several but they don’t work? Have you tried this one: http://wordpress.org/plugins/font-resizer/
Which plugins have you tried so far?
I made a note to start creating themes with “Accessibility” capabilities (which many do not exist). One of the things I will do is change from px font sizes to em sizing which is relative.
What I can probably do either today or tomorrow is update Encounters to use relative font sizes instead of px. This plugin http://wordpress.org/plugins/font-resizer/ worked for me when I did a preliminary test by changing the content to em sizes.