i dont know about flatties but with a lot of aquarium fish, and not going by colour differences, you vent sex fish., males have a more rounded vent where the fems are oval shape....
i also found this
Why do we have size limits?
In Queensland, there are limits on the size of #sh that can be legally taken for many species. There are minimum size limits and also some maximum size limits.
Size limits are typically based on biological research into each species' reproductive cycles. Minimum size limits generally allow #sh to spawn at least once and contribute to the population before they are taken.
However, in some species larger individuals contribute more to the population, which is why maximum size limits apply. For example, most barramundi begin their lives as males and later, as they grow larger, become females. A maximum size limit is applied to protect large females and to allow them to spawn.