[Brian]

The setting you're after exists in the Text tab of the Style inspector. Specifically, you want the "Resize to Fit" option. See attached screenshot for the control you're after; you may optionally want to uncheck the "Wrap to shape" box for those shapes, as well.

If you're going to be making a lot of diagrams that work this way, you may want to define a diagram style (via File -> New Resource -> New Diagram Style) you can then apply to later documents. The "Using diagram styles" page in the Help menu material also has more info on them.

Hope that helps!