set image dimensions upon theme activation

functions.php

<?php
/**
 * Hook in on activation
 */
global $pagenow;
if ( is_admin() && isset( $_GET['activated'] ) && $pagenow == 'themes.php' ) add_action( 'init', 'yourtheme_woocommerce_image_dimensions', 1 );

/**
 * Define image sizes
 */
function yourtheme_woocommerce_image_dimensions() {
  	$catalog = array(
		'width' 	=> '400',	// px
		'height'	=> '400',	// px
		'crop'		=> 1 		// true
	);

	$single = array(
		'width' 	=> '600',	// px
		'height'	=> '600',	// px
		'crop'		=> 1 		// true
	);

	$thumbnail = array(
		'width' 	=> '120',	// px
		'height'	=> '120',	// px
		'crop'		=> 0 		// false
	);

	// Image sizes
	update_option( 'shop_catalog_image_size', $catalog ); 		// Product category thumbs
	update_option( 'shop_single_image_size', $single ); 		// Single product image
	update_option( 'shop_thumbnail_image_size', $thumbnail ); 	// Image gallery thumbs
}