Zoom slider doesn't work in Outlook 2013
Located on the bottom right corner in outlook.
This is related to if you have the reading pane on or off.
If you have it off (no preview) then you can't adjust the setting.
If you turn on the Reading Pane (preview) then the zoom slider bar becomes active and you can use it.
To turn it on, select the View Tab, Reading Pane and make your selection for the right or bottom preview pane.
Now the zoom slider is active and ready to use.