There's no definitive answer to how wide your cross-country (XC) bars should be, as it depends on a number of factors such as your body size and riding style. That said, most XC bars are between 660mm and 680mm wide. If you're on the smaller side, you might want to go for narrower bars, while if you're bigger or more aggressive in your riding, wider bars might suit you better. Ultimately, it's best to experiment with different widths to see what feels most comfortable for you.