This feature is added on version 18.104.22.168, so make sure you are using the version 22.214.171.124 or above.
By default, our slider just calls the WooCommerce product loop thumbnail template. That means it’s the same size as your shop page products image. So if your theme has WooCommerce support you should able to change the image size from theme customizer. You can follow this article for details.
Or you can use our plugin filter hook to change it. For using this filter you have to add a parameter (woo_image) in our product slider shortcode.
Here is an example of the shortcode –
[wpb-product-slider posts="12" autoplay="true" woo_image="false"]
After adding the woo_image with its value false, add this PHP code on your theme functions.php file. Add your own image size on the code.