First you need to select/highlight all the layers. Then select all, you can press ctrl a on the keyboard or on the menu, select> select all to do so.
Then you will see the whole page is selected, press ctrl t, or go edit> free transform, the transform menu should be pop up.
Then make sure it is "scale" option, and the keep ratio box is click. In either width or height, type in 67% and hit ok. Now your page should be shrink down to the standard size. Please note that depends on your computer speed and how many layers you transform together, it may take a while for the page to shrink.
