There are plenty of species of chams that stay smaller, not all of them require 60+ gallon tanks. If you were going to get a cham I would suggest going with an easier to keep species, like a graceful cham, not overly flashy, but they are smaller and pretty easy to care for as far as chams go.
But think long term with the species you choose, are you going to be able to offer a large enough space if you do decide to go with a larger species? I know long term can be hard to grasp, but this is something you NEED to talk over with your mom, will you have space for a 90 gallon or larger for some chams, what about leachies are you going to have the space long term for them? If you ask your mom and youre limited on space the chewie might be a better choice for you. But like I said you seriously need to ask your mom on what HER limit is, your limit might be 200 gallons, hers may be 50, and that can seriously determine which species of these 3 you can safely keep.
But also handling, do you want a show piece or do you want a handling pet? If you want a good handling pet go with the chewie. If you dont mind being crapped on and bit at because they can be cage aggressive, get a leachie. And if you just want to look at it and go "ooo yur pretty", then consider a cham. Like I said there is more to it then just cage space. Also look at care, some chams prefer cooler temps and higher humidity, others prefer warmer temps mod humidity, others cooler and mod humidity. They can really vary, but not all of them are good beginner species. So I think you need to sit down and talk with your mom once you move, see how much space she will allow you to have. Then ask yourself is she willing to care for your reptile while youre at school. Or do you save your money and invest in a misting system. If you go on vacations will you have someone to care for them (obviously rhacs are easier to find pet sitters for since they can go bug free)? I think you need to take a step back and really think things over with your mom.