"This", the stock by attributes mod has no involvement in building the pulldown menu. As this thread is over 700 posts just on the mod, it would be a bad idea to extend its scope into other attribute-related queries too. Besides your question warrants a thread of its own.
But before you start one, could I suggest that you consider whether this really is a good idea. My experience suggests that changing to having a particular size as a default leads to a sharp and expensive increase in the number of orders returned for sizing issues, and hacked off customers who feel that the site is poorly designed.
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