Dear Xara...

A Straight Line has bounds of its containing area and shows a Width (W) and a Height (H) depending on its Rotation Angle and Line Width, ignoring its Cap type.
You kindly provide a Perimeter (P) measure that indicates its Length, regardless of its orientation.

Equally, when I change the Line to a Bezier Curve, I still get a P-value.

If I change to a Shape, I still get a P-value. Even for the most convoluted QuickShape.

Bug - for a (Smart) Spiral - Zilch. I have to Ungroup it to find the P-value.

The Smart Labeled Line 2 even generated a smart length but no P-value. No Smart Line does.
I have yet to find a Smart Shape that does.

Bug - the drag use of the Smart Control Points (Blue Circle) is disconnected from any Snapping action.

In the case of the Spiral, one cannot easily place the start of the Spiral at the Spiral Centre. One cannot Snap the Centre to the Grid, Guide or any Object.

Please make these Smart Shapes of yours actually fit into your product.