Try this

1. unzip and place the CustomZoom.dll in your CamBam plugins folder
2. It will appear in the Plugins menu as "Custom Zoom"
3. Click the GetZoom button to get a value for the current zoom setting
This gives you an idea of what to enter for the desired zoom, a lower value zooms out.
4. Enter a number in the lower text box for the desired zoom value then press the SetZoom button
CamBam will update the screen to the new setting.
Repeat step 4 until you get the desired size.
On my system (mm) the default zoom value is 3.779528 if you use inches it will be x25.4 making 96
It will remember the zoom value you entered until CamBam is closed.