The size guide feature (custom post type) was developed to add a size chart to a product page. This function can be activated in Theme Settings -> Shop and Size guides option is enabled at the bottom of the setting. A new size guide chart can be added in Dashboard -> Size Guides.
Each product has the option to choose the size guide among all the existing. In addition, there is the option to hide the chart and leave the content area allowing to add custom content in the popup window to each product.