functions.php

<?php
/**
 * Hook in on activation
 */

/**
 * Define image sizes
 */
function yourtheme_woocommerce_image_dimensions() {
	global $pagenow;

	if ( ! isset( $_GET['activated'] ) || $pagenow != 'themes.php' )
		return;

	$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
}
add_action( 'admin_init', 'yourtheme_woocommerce_image_dimensions', 1 );