Hi Sebastian,
You do not have to change any file in the function.php as replied here https://wordpress.org/support/topic/content-slider-deminsion First regenerated the thumbnail via https://wordpress.org/plugins/regenerate-thumbnails/? If you have upload the image size 962*390 before then images will be regenerated otherwise you have to upload the image once again with max size 962*390 to have the default slider size that our theme recommended.
Thank you!
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