Size isn't the only issue with a 100 gallon tank. Humidity and providing the proper depth of substrate are of utmost importance.
Providing a hot enough basking spot, preferably at the top of a Rete's stack is equally important.
While some may claim that an ackie makes a good beginners' monitor, most monitor keepers would tell you that monitors are not good beginner's pets. They do not tolerate husbandry miscues. While ackies may be smaller, they still require a lot of space for their size and a lot of attention to several important husbandry parameters.