thank you for your answer. Yes, I tried the “Font-Resizer” and, No, it did not work. I tried other plugins, and most of them did not work, neither.
When looking for other good solution I found the plugin “WP Accessability” (http://wordpress.org/plugins/wp-accessibility/). I brings a lot features which are needed to increase the accessabilty of websites for disabled persons. With this plugin, resizing is possible.
Unfortunately, this plugin in difficult to handle when you are not good in CSS (e.g. the div for the skiplink box has to be build in manually etc). On the other hand, the greyscale-modus is not working with Firefox.
If you can update “Encounters” with relative font sizes: great!