Im going to join a boxing club and have a few questions?

You don't have to worry, the won't become noticably bigger by boxing. Your arm strength will grow but not the m of your muscles, because boxing trains the effectivity of your muscles. The more you have from the start, the less your arms will grow, because every single fiber of your muscles hasa great potential for more effectivity. Your body won't start to develope new ones until your present ones are not trained to 100 %.