Hi again, Andrei.
As previously stated, we can’t help with requests like pop-ups or custom styling, but I can tell you that, by default, the Full size image is used: https://github.com/moderntribe/the-events-calendar/blob/4.6.2/src/views/single-event.php#L58
Therefore, I’d guess you have a customized version of /wp-content/plugins/the-events-calendar/src/views/single-event.php in your parent or child theme or some other code customization in place that is causing something other than the Full size image to be displayed.
Please look into this further and let me know what you find out.