Hello,
I can help you out with this. There is not size of the image set. It takes the full image you upload and fits it into the width of the content.
One way to make this change would be to add some css to your site to resize the image to what you would like.
Or you can follow our themer’s guide:
https://theeventscalendar.com/support/documentation/events-calendar-themers-guide/
Then you can move and edit this file:
/the-events-calendar/views/single-event.php
On that file you would like to look for this function:
tribe_event_featured_image
And you can change full to the size you would like.
This information from the codex can help you create a custom size if you like:
http://codex.wordpress.org/Function_Reference/add_image_size
Let me know if you have any follow up questions.
Thanks