Scenario: Pennyworth’s Haberdashery, a long-standing fashion retailer with multiple stores in Greater London, has recently acquired a clothing line in Madrid. As part of the integration process, they are merging systems with Microsoft Power Platform. Currently, Bruce Wayne is preparing to train his first machine learning model, and one of the input features consists of sweater sizes with values: XS, S, M, L, and XL. Question: What is the best approach Bruce should take to preprocess the sweater size input feature effectively for machine learning?