(see also: Shape Tools Overview)
The Parametric Shapes tool gives you direct control over the precise dimensions of several different shapes.
The default DesignCentral window does not offer much, but the real ability of the tool is in the Edit... button.
Each different shape in the Shape Dropdown has its own individual parameters you can edit. The Preview window will give you a quick preview of the result of the parameters on the left. The Parameters window show you exactly which option corresponds with the measurements on the left. For example, H1 references the tail width, as seen in the Parameter window.