Globally setup size guide for all products
You can setup same size guide for all the products then you can manage it in general for overall store products for that Go to admin panel > Appearance > Customize > WooCommerce > Product Single > Size guide.
Individual setup size guide for single product
Product size guide is always expected by your customers while purchasing clothes from your website and so this product size guide feature is must to product pages. To add this option, you first need to navigate admin panel > Products > All products > Edit specific product >Hongo Product Settings at bottom > select single page settings and go to size guide option. Then, add required details related to size guide text, title, and content like below image.
Once you have entered all details, to see size guide popup on product page, click on “size guide” like below image.