One easy way to implement that using the same template(s) in both the Palette and the main Diagram is to set the Diagram.initialScale on the Palette to be a value larger than 1.0.
The Canvases sample sets that property on the Palette to 0.5 in order to make the items smaller than they will be in the Diagram.
Spacing between items in the palette is determined by the Palette.layout. By default that is a GridLayout, so you just need to set the GridLayout | GoJS API and GridLayout | GoJS API and perhaps other properties.