the maximum height of the aquariu7m is proportional to the thickness of the material that it is constructed from.. i don't know the numbers offhand, but as you get taller, the thickness of the walls must increase. you can look at a number of custom acrylic and glass tank manufacturers and see what kind of dimensions you can get with what thickness.
also keep in mind, that unless you have access from the top, that is, you can stand along the top or side of the tank, such as in steveweast's case, you will have difficulty reaching the middle and bottom of the tank if it is anything deeper than the length of your arm, and no taller at the lip than your armpit.