How do I dimension in a viewport?

1. Switch to paper space and set up the viewport.
2. Choose Dimension and click the model points through the viewport with snap — the snap reaches the model geometry. Or click a line, circle or arc in the viewport as an element.
3. Position the dimension line on the sheet and click.

The dimension shows the model size (the paper distance is combined with the viewport scale) and follows the model geometry. Such dimensions are always driven. If the two points came from different viewports or one from the sheet, the paper size applies.