My biggest tank is 50 gallons and the biggest fish (who lives in there, along with 4 other goldfish, 9 danios, 2 WCMM, 3 bronze cories, 1 weatherloach, and 1 pleco) is my Black Moore. He is still pretty small at only 3" and with his tail he is 5". I've had him for almost a year now (along with the other goldfish) but I don't think he's grown much...maybe about 1/2 cm or so.
Edit: Wow! That tank is huge...and beautiful!!! What kind of fish and how many of them do you have in there?
Biggest tank is a 6 x 2 x 2 - 680 litres, and my biggest fish is probably my Synodontis Angelicus at about 8.5". I have just purchased 3 new Pimelodae catfish which are between 6 and 7". My Black Ghost Knife is also around 7".