Its not too big. Just remember that the bigger the tank, the harder it is for your geckos to find food, especially if its insect food like crickets. When it comes to cresteds its not so much and width you want in a tank, its height. Cresteds like to climb so having a deep tank is better then having a wide tank. Usually about 10 gallons per gecko is enough to keep them happy. Ex: 2 geckos = 20 gallons etc. But as long as you mist the tank 2 times a day to keep humidity up, 75 gallons should be plenty enough for them.