I agree that a measuring tool would be nice but you can usually get around it a couple of different ways.
Most of the time you can position openings in mat designer to give exact intersect points for the start and end points for lines to be drawn in PathTrace (e.g. if you need to construct a trapezoid, you can start in Mat Designer with a rectangular opening of the correct dimension for the parallel sides, then mark the start and end points for the other two sides by positioning additional openings [the alignment feature is useful in this regard]. Then in PathTrace you just draw two straight lines from these points, delete the positional openings, and you're done).
Obviously you can also scan in an image of the desired shape or dimension, opening it in PathTrace with the specified size. For instance you can scan in a jpeg of a 12" rule and then use this as a gauge within PathTrace. A bit crude but usually close enough.