In the menu on the left, you will see 'size charts'. Via this tab, you can 'add a size chart' or make adjustments to an existing one.
Choose a title, for example 'size guide'. On the right, you will see a short code, a unique code for each size chart you create. You can copy and paste this short code wherever you want.
Scroll further down and start creating your own size chart. Use the green + button to add columns and rows, and the red - button to delete them.
When you have finished, click on 'publish' or 'update' at the top right of the page. Your size chart, with columns and rows of your own choice, is now added to your shop and will look something like this ...
Note: your size chart will not look exactly the same. This is an example of a size chart implemented on another website's branding with custom CSS code.