menu View -> Size and Position window --> select your two (or more) shapes --> type the new width (or height) in the appropriate box.
It is always hard to find the button or window in office since version 2007 / 2010..., actually I really hate the new tool bar, more prefer office 2003 style, but we have to move on with MS, :-(
so, I posted the screenshot here just for finding it easily next time.