One of the most common complaint raised against online stores is size related issues. Sizes such as small, medium, large etc can vary widely when it comes to actual fit.
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. Since customers are more likely to buy when they are sure the product would fit, this can boost your sales significantly.