Boas don't require that much room. I have all my snakes in AP T10s except the carpet who will be getting a T12. And they all seem happy. Boas aren't really that active so they don't need huge caging. You want something that needs a lot of room, get a monitor. My Nile Monitor needed a 8x4 cage and my Savannah needed a 6x3 and those were a bit on the small side. But back to the subject, like others have said, unfortunately its your parents house so its their rules. Now maybe you can sweet talk them later down the road or switch pet types. I wanted snakes so bad as a kid but my mom was scared of them. I ended up getting into monitors instead. Then later I switched to big snakes. The monitors were the gateway for me with my mom. Once she was used to my 5' nile, she wasn't so scared of snakes. I then started with ball pythons as they were small and hid a lot. Once she got used to those, I got boas lol. Thing to remember is regardless what you get, it is going to be with you a long time and is going to need caring for. If you cant commit to 20 years+, then my advice is don't get it. Rehoming a larger snake is not an easy task and also imo, its not much fun for the snake either.