When creating configurable 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 >> Product Tab

Set configurable SKU to AttrOptionValue and then Configurable name to Name<options> if you'd like the value to added to the end of the SKU (screenshot is clickable)