Hello Carl, thanks for getting in touch!
Yes, there absolutely is a way to change the featured image size. The Events Calendar simply uses the default WordPress Featured Image functionality, so it pulls the same Featured Image size settings that are already on your site.
You can change the settings of your featured images in WordPress under Settings > Media. Change the dimensions of the Medium Size and this will take effect.
Otherwise, yes, CSS would be an alternative and you can target the image container using something like:
.tribe-events-list .tribe-events-event-image {
width: 20%; /* or whatever size */
}
Does this help answer your question? Please let me know. 🙂
Cheers!
Geoff