I´ve done it by changing the sizes of the theme / WP
…../wp-admin/options-media.php
After setting the sizes you have to regenerate the thumbnails of the images you allready stored. This is easy with a plugin like
https://de.ww.wp.xz.cn/plugins/regenerate-thumbnails/
You can register a custom thumbnail size using the add_image_size function – https://developer.ww.wp.xz.cn/reference/functions/add_image_size/ (you’d need to add code to the functions.php file in your theme, or find a plugin to allow you to create image sizes on the fly).
Once you have registered an image size, you can use the name you gave it in the thumbs parameter – e.g.:
[child_pages thumbs="mycustomthumb"]
