Standard (or Medium) width is the default fit for most cycling shoes and works well for the majority of riders. If you've never had issues with shoes feeling too tight across the ball of your foot or too loose around your heel, Standard is likely your best starting point.
The key sign that Standard isn't for you is foot discomfort during rides. If you experience numbness, tingling, or hot spots on longer rides, your shoes may be too narrow—try a Wide fit instead. On the flip side, if your heel slips or your foot slides around inside the shoe, a Narrow fit might give you a more secure hold.
One important tip: don't size up in length to solve a width problem. A longer shoe won't fix forefoot squeezing and can create other fit issues. Instead, move to a wider width while keeping the same length. Brands like Shimano and Lake offer multiple width options, while Italian brands like Sidi and Fizik tend to run narrower even in their Standard widths.