If you sell clothing items that require customers to know their size before they make a purchase, you can add a custom size chart to your product page. This instruction shows you how to set up the size chart.
First, you need to create a page for your size chart content. From your Shopify admin, go to Online Store > Pages then click Add page
You can enable the size chart on the product page by navigating to the Pop-up size guide block. There is an option named Select Page for Size Guide popup, and here’s to choose the page that you just created. When clicked, it opens a popup that represents the page’s content: