You have to unlock the shape's protection first. After that you will be able to resize the shape. Here is how it is done in Visio 2013:
First you need to enable the Developer Tab in your Visio. Check how it is done here. After that mark your shape with the mouse and go to the Developer Tab, click on protection and uncheck width and height. Now you should be able to resize the shape. It is done similar in Visio 2007, click here.