Sure thing Alison! The file in question is this one in Events Calendar Pro:
/src/views/pro/widgets/list-widget.php
There is also a version of this file in The Events Calendar, at this location:
/src/views/widgets/list-widget.php
It’s the same path without the additional /pro/ subdirectory.
Making a customized version of the former file should work well for you, but if you notice that some changes aren’t taking effect, then add a customized version of the latter file too. So that you’d have these files in your theme, exact duplicates of each other:
/tribe-events/pro/widgets/list-widget.php
/tribe-events/widgets/list-widget.php
I hope that helps!
— George