You are in luck Marlies23, I have been struggling to work out how to do this and eventually I found the time to take a look in the eventlist css file within the plugin zip and found that if you add this to your custom css you can change the font size.
.event-details {
font-size: 1.1em;
the default value is 0.8em but I found 1.1em was a happy medium, hope this helps you out
example on the society site I maintain https://www.iow.bcss.org.uk/?page_id=38