If your underwear is constantly riding up or slipping down, it's probably not the right size. If it feels constricting or uncomfortable in any way, it's also likely that you need a different size. Another way to tell if your underwear is the wrong size is to pay attention to the waistband. If the waistband is constantly digging into your skin or causing irritation, it's probably too tight. Paying attention to these signs will help you ensure that you're always wearing comfortable, well-fitting underwear.