Why do the users not enlarge the font-size in their browsers? A quick way to do so is holding control while using the scroll wheel of the mouse, this usually works quite well.
No, this does not work well at all. 
It does with my Mozilla/Firefox browser as those overwrite fixed font settings as opposite to M$ Internet Exploder
1. This is a global setting for the session in Internet Explorer. Meaning this will affect all the other websites they go to.
I know that, but it is a very simple solution that is not hard for a user to do.
2. With all the other browsers, it need to be done each time. Again, no good.
What I am looking for is a setting in Webmail for setting the font size.
That is not there I believe on the version of Horde that is used on version 7, as this command
grep -H -d recurse font-size /home/httpd/html/horde/*
only showes fixed font-sizes. If you want to change them all be my guest, but be aware that they will be removed when horde is upgraded.