Set sizes for shapes

I would like to be able to set a size for shapes, either in pixels or in/cm.

For example, I want to create a 4" circle. Currently, I have to make a 4" square canvas design and then draw a circle to the edges, which is quite fiddly.

Instead, I want to create a canvas shape and then set a circle in it, and click to re-size to 4".

.. or have the option to make round canvas designs from the beginning, without needing to draw an outline.