In my last quick tips video I showed a tip for making a super fast square (or circle). Well, it turns out when I said that anyone who knows of a faster way should tell me. And vimeo user Jorge Sorbara did. So double click the tool to make a shape and then hold option and double click the chain link next to size. This will set the y value to whatever the x value is. And it works on other properties with that ratio chain link, like scale.