Dimensions in Viewports

Dimensioning within a viewport in PaperSpace will provide dimensions of the object's 2D projection onto the monitor screen, not of the object itself. The dimensions will be scaled correctly according to the scale of the view, but misleading dimensions can result if the monitor screen is not exactly parallel to the object.
To dimension an object in non-parallel views within a viewport, you must enter the correct measurements manually in the Inspector Bar or in the dimension's Properties. When non parallel views are used, dimensioning is best done in Model Space.
See Viewports.