[Theme Horse Support Team]

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!