When creating matrix items, how do I get the size value added to the SKU of the simple item? 

By default, it currently adds a number in sequential order for each size to the simple item SKU.

Go to Settings >> Admin Settings >> General Settings >>Advanced Settings>>Products Product Settings Tab.

you can Auto Assign SKU's for matrix products by choosing "Yes" option for that field.

Enter number ranging from 100- 600 which the system can assign automatically to your matrix SKU's and Set Variant SKU Auto Number to Counter please see the  screenshot below: