The Events Calendar displays the full image size from the Media library, when you click on an individual event. I foud that rather uggly, so I diged up on how to modify the image size on the page.

This can be done via the functions.php file in the current theme.

Add the following lines to the end of the file:

function avf_single_event_post_thumbnail_size($size)
{
    if ( is_singular('tribe_events') ) {
        $size = 'full'; // <-- Change to your desired image size.
    }

    return $size;
}

You see that the original parameter $size is set to ‘full’. You can change this to other standard image sized (large, medium, small). The images on the Events list use a medium setting (just for comparation).

Save the file and reload the site…