Hey Dave,
Welcome to the Pre-Sales Channel and thank you for reaching out! The featured image size is really dependent upon the layout of your event pages, largest available size of image available, etc. You can use the default WordPress image sizes by calling in ‘thumbnail’ or ‘medium’ for example. (see screenshot below)

If you need to create a custom size for your event featured image, you could do that in your theme’s functions.php file and call that size for all your event’s featured images. More info on that can be found here.
I hope this helps answer your question and let me know if you have any additional comments. Have a great Thursday and thanks again for contacting our Pre-Sales Channel.
Cheers!