The information provided by size structure analysis of the pelagic ecosystem can be summarized by an index of size diversity. However, a problem emerges when species-diversity indices are converted into size-diversity indices due to the arbitrariness introduced when choosing the width of the size classes. In this paper I analyse this handicap and show how it is overcome when an index appropriate to continuous variables (m_p) is used, m_p is also a sensitive index to natural fluctuations as displayed by its evolution during a summer time series analysis in Mediterranean coastal waters.