The generate_page_header_default_size filter allows you to change the default size of your featured images.
Default (string): full
The default available sizes are: full, large, medium and thumbnail.
You can create more sizes using the add_image_size() function.
Usage
Please refer to the Using Filters article to learn how to use this filter.
generate_page_header_default_size
generate_page_header_default_size