Hey Vizzual,
first of all, thank you for the purchase!
The shrinking header is a part of the theme’s design, and therefore does not have a ready back-end switch. However, the custom solution is pretty simple – removing the jQuery code that animates the entire thing, so I thought I might prepare a file for you. Please download this archive, unzip it, and upload the contained file into the theme folder (/wp-content/themes/Kanso/), replacing the existing file. Let me know if it works!
Sincerely,
Andrii / Satori Studio