how to change DefaultSize of viewport
The viewport is basically the size of the HTML DIV element that is hosting the GoJS Diagram. So you need to figure out how to size that element the way that you want.
Note that if the window is not being resized, there is no notification that the DIV may have changed size, so you’ll need to explicitly tell GoJS that the DIV has changed size: Resizing Diagrams -- Northwoods Software. It’s not a problem if the window changes size, because GoJS does listen for that event.