Size guide feature was introduced in our theme from 3.6 version so to use it you need to have the latest version of the theme and updated Basel core plugin. This function can be activated in Theme Settings -> Product Page -> Size guides. Then, you can create new tables in Dashboard -> Size Guides. Each size guide table you can attach to particular product categories while editing them or to some specific products when you edit these products. You can also put any content like text or pictures that will be displayed before the size table.