This enables custom image sizes for WooCommerce in the Customizer even when the theme has declared the images sizes. This works with the Storefront theme.

Enable WooCommerce image sizes in Customizer.php

<?php
/*
Plugin Name: Custom Image Sizes for WooCommerce
Plugin URI: https://themebynumbers.com/
Description: Let's you customize WooCommerce product image sizes from the Customizer
Version: 0.1
Author: Mikey Arce
Author URI: https://themebynumbers.com
Text Domain: custom_image_sizes_for_wc
Domain Path: /languages
License: GPL2
*/

if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

class Custom_Image_Sizes_WC {
	
	public function __construct() {
		add_action( 'customize_register', array( $this, 'cisfw_woocommerce_image_size_options' ) );
		add_action( 'after_setup_theme', array($this, 'set_custom_image_sizes'), 11 );
	}

	public function cisfw_woocommerce_image_size_options( $wp_customize ) {

		if ( wc_get_theme_support( 'thumbnail_image_width' ) ) {
			$wp_customize->add_setting(
				'custom_woocommerce_thumbnail_image_width',
				array(
					'default'              => 300,
					'type'                 => 'option',
					'capability'           => 'manage_woocommerce',
					'sanitize_callback'    => 'absint',
					'sanitize_js_callback' => 'absint',
				)
			);

			$wp_customize->add_control(
				'custom_woocommerce_thumbnail_image_width',
				array(
					'label'       => __( 'Catalog/Shop Thumbnail image width', 'custom_image_sizes_for_wc' ),
					'description' => __( 'Image size used for products in the catalog.', 'custom_image_sizes_for_wc' ),
					'section'     => 'woocommerce_product_images',
					'settings'    => 'custom_woocommerce_thumbnail_image_width',
					'type'        => 'number',
					'input_attrs' => array(
						'min'  => 0,
						'step' => 1,
					),
				)
			);
		}
		if ( wc_get_theme_support( 'single_image_width' ) ) {
			$wp_customize->add_setting(
				'custom_woocommerce_single_image_width',
				array(
					'default'              => 600,
					'type'                 => 'option',
					'capability'           => 'manage_woocommerce',
					'sanitize_callback'    => 'absint',
					'sanitize_js_callback' => 'absint',
				)
			);

			$wp_customize->add_control(
				'custom_woocommerce_single_image_width',
				array(
					'label'       => __( 'Single Product image width', 'custom_image_sizes_for_wc' ),
					'description' => __( 'Image size used for the main image on single product pages. These images will remain uncropped.', 'custom_image_sizes_for_wc' ),
					'section'     => 'woocommerce_product_images',
					'settings'    => 'custom_woocommerce_single_image_width',
					'type'        => 'number',
					'input_attrs' => array(
						'min'  => 0,
						'step' => 1,
					),
				)
			);
		}
		$wp_customize->add_setting(
			'custom_woocommerce_gallery_thumbnail_image_size',
			array(
				'default'              => 100,
				'type'                 => 'option',
				'capability'           => 'manage_woocommerce',
				'sanitize_callback'    => 'absint',
				'sanitize_js_callback' => 'absint',
			)
		);

		$wp_customize->add_control(
			'custom_woocommerce_gallery_thumbnail_image_size',
			array(
				'label'       => __( 'Single Product Thumbnail image width', 'custom_image_sizes_for_wc' ),
				'description' => __( 'Image size used for the thumbnail gallery on single product pages. These images will be cropped into squares.', 'custom_image_sizes_for_wc' ),
				'section'     => 'woocommerce_product_images',
				'settings'    => 'custom_woocommerce_gallery_thumbnail_image_size',
				'type'        => 'number',
				'input_attrs' => array(
					'min'  => 0,
					'step' => 1,
				),
			)
		);
	}

	public function set_custom_image_sizes() {
		
		// Filter the size of the WooCommerce Thumbnail
		// These are the product images that appear on the Catalog (Shop page)
		add_filter( 'woocommerce_get_image_size_thumbnail', function( $size ) {
			
			$woocommerce_thumbnail_width = get_option('custom_woocommerce_thumbnail_image_width');
			$woocommerce_thumbnail_cropping = get_option('woocommerce_thumbnail_cropping');
			$cropping = get_option( 'woocommerce_thumbnail_cropping' );

			switch ( $cropping ) {
				case 'uncropped':
					$custom_height = '';
					$custom_crop   = 0;
					break;
				case 'custom':				
					$width          = max( 1, get_option( 'woocommerce_thumbnail_cropping_custom_width' ) );
					$height         = max( 1, get_option( 'woocommerce_thumbnail_cropping_custom_height' ) );	
					$custom_height  = absint( round( ( $woocommerce_thumbnail_width / $width ) * $height ) );
					$custom_crop    = 1;
					break;
				case '1:1':
					$cropping_split = explode( ':', $cropping );
					$width          = max( 1, current( $cropping_split ) );
					$height         = max( 1, end( $cropping_split ) );
					$custom_height  = absint( round( ( $woocommerce_thumbnail_width / $width ) * $height ) );
					$custom_crop    = 1;
					break;
			}

			return array(
				'width'  => $woocommerce_thumbnail_width,
				'height' => $custom_height,
				'crop'   => $custom_crop,
			);
		} );
		
		// Filter the size of the WooCommerce Single Product page image.
		add_filter( 'woocommerce_get_image_size_single', function( $size ) {

			$woocommerce_custom_single_product_width = get_option( 'custom_woocommerce_single_image_width' );
			
			return array(
				'width'  => $woocommerce_custom_single_product_width,
				'height' => '',
				'crop'   => 0,
			);
		} );

		// Filter the size of the WooCommerce Single Product page thumbnails.
		add_filter( 'woocommerce_get_image_size_gallery_thumbnail', function( $size ) {

			$woocommerce_custom_single_gallery_thumbnail_width = get_option( 'custom_woocommerce_gallery_thumbnail_image_size' );
			
			return array(
				'width'  => $woocommerce_custom_single_gallery_thumbnail_width,
				'height' => $woocommerce_custom_single_gallery_thumbnail_width,
				'crop'   => 1,
			);
		} );

	}
}
new Custom_Image_Sizes_WC();