How to implement add_image_size in a plugin

I created a plugin and I need to use add_image_size in it.

I know how it works in the functions.php file and for a theme, but how can I implement this in a plugin?

I want new images uploaded by WordPress users to be smaller for mobile screens by

add_image_size( 'wp_small', 60, 75, true ); // mobile

I read that you need to use init or admin_init but I don’t know how to implement this.

Just call this function in the init action. This action is fired for both frontend and backend. So it should look like this:

add_action( 'init', 'wpse4378_add_new_image_size' );
function wpse4378_add_new_image_size() {
    add_image_size( 'wp_small', 60, 75, true ); //mobile
}