The presentation of nutrition information on a serving size basis is a strategy that has been adopted by several countries to promote healthy eating. Variation in serving size, however, can alter the nutritional values reported on food labels and compromise the food choices made by the population. This narrative review aimed to discuss (1) current nutrition labelling legislation regarding serving size and (2) the implications of declared serving size for nutrition information available on packaged foods. Most countries with mandatory food labelling require that serving size be presented on food labels, but variation in this information is generally allowed. Studies have reported a lack of standardisation among serving sizes of similar products which may compromise the usability of nutrition information. Moreover, studies indicate that food companies may be varying serving sizes as a marketing strategy to stimulate sales by reporting lower values of certain nutrients or lower energy values on nutrition information labels. There is a need to define the best format for presenting serving size on food labels in order to provide clear and easily comprehensible nutrition information to the consumer.