I am not sure if there is an explicit document that details how to make a custom template. The Plant3d help documentation on Iso should help you out. You will need to go through the project setup in Plant3D to set the north arrow.
In brief, north arrow can be changed using the drawing setup UI. Text height can be changed by using switches 4 and 27 in the opl file. There is no UI exposed currently to set these two switches.
Switch 4 to control the text size of Dimensions, Coordinates, Message Text, etc.
Switch 27 to control the material list text.
Unfortunately, we do not have any good whitepaper on this.
A template can be created two way, through the 3D Plant Isogen settings, or Alias OPT editor.
I do have a custom template with custom MTO etc. See attachment.
Change the path in the FLS file to yours path.
You can use the Alias editor tool for Isogen configuration. Just search the internet for Alias Isogen editor, and you will find several document for download.