You need to create a child calendar theme. See https://time.ly/document/customize-calendar/create-new-calendar-theme/ – Like in that example, I’ll assume you’re using the name ‘mytheme’ for the dir. Feel free to name it whatever you like. DO NOT ACTIVATE IT till you make any changes.
Once you’ve done that, copy …../wp-content/plugins/all-in-one-event-calendar/public/themes-ai1ec/vortex/twig/agenda-widget.twig into …../wp-content/themes-ai1ec/mytheme/twig/ (if the directory doesn’t exist, create it).
You then have to edit that file you just copied into mytheme/twig/ and look for the following line:
{{ event.get_runtime( 'filtered_title' ) | truncate | raw }}
Replace it with:
{{ event.get_runtime( 'filtered_title' ) | raw }}
Once you’ve made the change, then you can activate the new calendar child theme you’ve created.
If you wanted it to truncate at 60 characters, you should be able to pass truncate a number (it’s 35 characters by default).
eg:
{{ event.get_runtime( 'filtered_title' ) | truncate (60) | raw }}
If you want to know more about twig, see: https://twig.sensiolabs.org/documentation
Hope this helps.
Note: Ai1ec now caches CSS and PHP templates (created from the twig files) in …../wp-content/plugins/all-in-one-event-calendar/cache/ – so if you make a change, you might need to clear that directory. This directory needs to be writeable by the webserver for this to work.