okies...dont say you have not been warned and told what is the best interests for the urchin....Really trying to help you here dude...take a step back and listen to the advise given in this thread, it will help you and the urchin..
It's also not just a case of how big it gets but what it needs to support it. You don't have enough rock and you don't have a big enough tank I'm sorry to say. I really would suggest you take it back.