Let me put it this way. In 4th ed, Conjuring the Mystic Tower was a CrTe35 spell with the same dimensions. They just didn't create a new spell from scratch. And technically, the spell is correct. Reducing the spell by one magnitude, you would get a smaller building. The spell could be reinvented at the same level to be bigger. But to create a structure of variable dimensions and shape would probably add magnitudes to the spell. So it can be explained away by, whoever used the guideline first thought a tower of those dimensions was ideal.