“px” units should work. I just tried it, setting the width and height of an HTML DIV element adjacent to the Diagram’s DIV, and also creating a Node that was exactly the same width and height. Positioning them next to each other showed that they were the same size, even after scrolling or zooming the page.
Of course scrolling or zooming the diagram would cause the node to appear at a different size and position, but I assume that is not what you are concerned about. Absolute positioning within the viewport is a sample Diagram that doesn’t allow scrolling or zooming.