Without being a killjoy, just be aware there's no fishing allowed from the ferry landing and jetties at Half Moon Bay, but you could fish from the rocks upriver of the boat ramp, and the rock wall on the Little Bucklands side of the marina 'hard' area (where the boats are pulled out for repair).
Good on you for asking about the size and catch limits. Further to PJC's post, the measure is from the tip of the nose to the v of the tail.
Grab yourself a free measuring sticker from most tackle shops.
Fisheries officers regularly inspect at Queens Wharf (CBD) and at Bucklands Beach, Half Moon Bay etc. Which is great to see.
Along the rock wall west of the Auckland Harbour Bridge is a popular shore spot and good for snapper, kahawai etc at hte right time of the year and tide.
Good luck.