Ah yes, that’s a change we’ll need to make in the plugin.
I’ve made it here for WPSP 1.2, so you can make this change in your files if you’d like: https://github.com/tomusborne/wp-show-posts/commit/02f0cf276ae35eb6f35545a69322da447793658c
Then you can do this:
add_filter( 'wpsp_default_image_size', function( $size, $settings ) {
if ( 123 === $settings['list_id'] ) {
return 'your-size-name';
}
return $size;
}, 10, 2 );
You’d just need to update 123
to the ID of your list.