I have heard of people using 32 qt for one or two adults just fine. But that also depends on the dimentions of the actual tub. There just has to be the proper sizing to allow enough floor space. As long as you can heat it properly and the gecko doesn't have trouble hunting the food if it is young you should be fine with that size. If it is young and have trouble eatting becuase of the open space just give it a smaller tub until its big enough.
If you wanna house more then one make sure they are roughly the same size and only one male. Best to have multiple females only unless you want the chance they will breed. And just make sure they are all getting the right amount of food.
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