Small, medium, and large: These sizes can vary widely when it comes to actual fit, and that can be a significant challenge if you sell clothing.
The problem is that whether you sell apparel with more generic or even numerical sizes (2,4,6, or 38 waist, etc.), none of them are completely standardized.
Having a size chart is the best way to resolve the problem, and it can help with conversions, since customers are more likely to buy when they are sure the product would fit.