Hi Phil.
If I understood your issue correctly, please try this custom CSS:
body.single-tribe_events iframe[id^="eventbrite-tickets"] {
min-height: 370px;
}
You can add custom CSS code via your child theme’s style.css file or via a plugin like Simple Custom CSS or, my favorite, Jetpack. Jetpack supports regular CSS and the LESS and Sass pre-processors.
A preview of the customization:

Let me know how it goes for you!
(Update: Look what I found! Another way to do it — via PHP instead of CSS: https://theeventscalendar.com/knowledgebase/changing-the-height-of-the-eventbrite-tickets-iframe/ )