How can I get a -Select- option in a product reference dropdown to force the user to select an option?

From what I gather, the way to create product attributes in Commerce is to create a separate product for each attribute, and then reference those products from a product node.

So, say I am selling a T-Shirt, with sizes S, M, L. I created a product reference for S, M, L, and reference them from my product node.

My select list for the product shows S, M, L, these are required fields.

Required fields should never have a default. In this scenario the default is 'S', where the default should be - Select -, which prompts the user to select a size option.

How can I get a -Select- option in a product reference dropdown to force the user to select an option?