Go to Administration > Stock > Sizes
Click New Size Scale.
Enter a Size Code. This can be upto 15 characters in length.
Enter a Description.
Click OK.
To add sizes to the size scale click New Sizes in the lower grid.
Enter a size. This can be upto 20 characters in length.
Enter a sort code. The sort code determines the order in which the sizes will be displayed. If no sort order is entered the sizes will be displayed in alphabetical or numerical order.
If you would like the sizes displayed in the order of Small, Medium, Large the sort order for each could be 5, 10 & 15.
By leaving a gap between the sort numbers it allows you to 'slot in' another size if the need arises.
Example: A new size may be available in Small Regular. If you wanted it to appear in between Small and Medium you would give it a sort number of 6, 7, 8 or 9.
Click Ok to add the size.
Click New Sizes to add more sizes to the size scale.
The Size Scale can then be applied to a Stock Item.
See https://datatecnix.freshdesk.com/solution/articles/1000206782-how-to-add-update-a-stock-item