You can enter any additional information about sizes to be displayed in the matrix view in the Web Storefront becuase size details will show the line break tags on the mobile devices, however on a browser they look great.
Example of size descriptions with international size scales:
To enter size description go to Items -> Manage Items
Go to Settings -> Items -> Variant Dimensions
Select the Dimension2 (Size) tab
Edit the size you want to add additional information for.
In the Size Description field write a <br> between the values you wish to be separated to a new line.
For example to show the size USA L in the example above with 2 additional descriptive lines:
Write: USA L<br>EUR 42<br>UK 12
You an also sort the order your sizes appear in the catalog - How to Sort Size Scale