Hi there Stanislas,
Thanks for reaching out and for the kind words about our products 🙂
The image size by default in events is ‘full’ which is actually the image original size (read more about it here: Post Thumbnails). So you can just upload the image in a smaller size or use a snippet to override the output of the function tribe_event_featured_image which is the one that handles the featured image for single event pages.
Please let me know if the above helps,
Best,
Nico