This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
It is not "considered" wrong, it is incorrect. In the body of the setSize method, "size" refers to the parameter, not to the instance variable. So what that assignment does is assign the parameter to itself - in other words, it does nothing. Especially the value of the instance variable of the same name is not changed.