WordPress | Function | Setting reserved image sizes on theme activation

set_image_sizes.php

<?php
// Setting reserved image sizes on theme activation
add_action( 'after_switch_theme', 'enforce_image_size_options' );
function enforce_image_size_options() {
    update_option( 'thumbnail_size_w', 160 );
    update_option( 'thumbnail_size_h', 160 );
    update_option( 'thumbnail_crop', 1 );
    update_option( 'medium_size_w', 660 );
    update_option( 'medium_size_h', 440 );
    update_option( 'medium_crop', 1 );
    update_option( 'large_size_w', 1920 );
    update_option( 'large_size_h', 1920 );
}