How to set setZoom to make the global viewport zoom instead of just the current window

As mentioned above, I want to set the global image size using the setZoom method

If you are looking to perhaps set an initial zoom on ALL viewports, then from the ViewportGridService you would grab each viewport and set the zoom (i.e. camera) from there. However, if you are looking to synchronize the zoom between viewports then you would use SyncGroups in a custom hanging protocol. The mpr hanging protocol has an example of sync groups for window level.

Hope this helps.