If you are selling items with a style code and variants, for example: color and size variants of a fashion style code, then the sizes of the item are displayed in alphabetical order by default on the "Matrix" view.
If an item comes in sizes S, M and L - they will be displayed by default in the order of L,M,S (alphabetical).
You can modify the order that your sizes are displayed in on the matrix view.
A note about Size Scales -
If you sell different types of products that come in multiple size types, you can create a "Size scale" for each size type. Shoes, clothing in S/M/L, clothing in numerical sizes, and men's shirts all come in different size scales. You don't want shoe sizes displaying in the matrix view for shirts or clothing. If you need size scales see this article to set them up: How to use Size Scales for Matrix items. You may then sort the sizes within each scale as needed.
Go to Settings -> Items -> Variant Dimensions, select "Dimension2 Group" tab.
The size scales you created will be listed.
Click on the size scale you wish to sort. The SortIndex for the sizes will be zero.
Select Integration Export. This will export a CSV file in the required format.
In the exported CSV file, enter a numerical ordering in the SortIndex column as you wish the sizes to be sorted. In this example, Pepperi would sort the letter sizes in alphabetical ascending order by default, however this will display the sizes in the opposite of the desired order in the Order Center. Modify the CSV file accordingly (highlighted below).
Select Import from the menu, and you will be directed to the APIDimensions2 upload process. Browse to your file.
Once it is loaded successfully, the Sort Index numbers will be updated in the list and your sizes will show in this order in the Order Center: