When considering what size of luggage to purchase, you are engaging in:

When deciding what size of luggage to purchase, you are engaging in capacity planning. This process involves determining the capacity needed to meet the anticipated demand for products or services. In this case, selecting the appropriate size of luggage is about ensuring that it can accommodate your travel needs, which reflects the underlying concept of capacity planning. You are assessing how much space is required based on your potential travel needs and the items you plan to carry.

Demand forecasting, on the other hand, relates to predicting future customer demand, which does not directly apply to personal luggage choices. Inventory management typically involves overseeing stock levels of goods, applicable more in a commercial context. Product selection could seem relevant as it involves choosing among different offerings; however, it does not capture the capacity aspect that is intrinsic in assessing what you can carry with the luggage, making capacity planning the most fitting concept here.