Per the following Microsoft article it is not possible to set a fixed zoom in Outlook 2010 under the message preview pane or full message view.
Although there is a feature request to Microsoft we would like to write a custom vm macro (or any other coding you recommend) which would allow for the zoom to be set permanently in Microsoft Outlook 2010
We have an end user who would like to be able to set the zoom scale to a value (e.g. 150%) and for every message that is previewed in the preview pane or opened in full screen in the message view the zoom will be set to that value.
http://office.microsoft.com/en-us/outlook-help/make-previewed-items-larger-or-smaller-in-the-reading-pane-HA010355101.aspx?CTT=1
