How do I indicate that a specific size / color of a product is sold out?
Assuming you are not using Inventory Control, you can mark the SKU associated with the size in question as Unavailable. To do this, Navigate to Products / Product Name and click the SKU Detail link under the Attributes banner. Next, click the Edit button in the SKUs banner to access the Edit SKUs screen. Once there, simply uncheck the Available box for the appropriate SKU.
If you are using Inventory Control, the SKU will become sold out if your Depletion Status is set to Sold Out and the SKU has an inventory of 0.