How can I change corner radius in PowerPoint?

If you don't mind getting a little VBA under your fingers, you can do this "by the numbers".

Create a shape and adjust the yellow handle so the curvature of the corners looks good to you.  Make sure the shape is still selected before moving on to ...

Start the VBA editor, make sure the Immediate window is visible and type this into it:

? Activewindow.Selection.ShapeRange(1).Adjustments(1)

VBA will display the value of the adjustment on the next line.  Let's say for argument that it's 0.11052

Next, select another shape that you want to match to the first and do this in the Immediate window:

Activewindow.Selection.ShapeRange(1).Adjustments(1)=0.11052

Or select several shapes and set them all at once like so:

Activewindow.Selection.ShapeRange.Adjustments(1)=0.11052

There is no way to specify a value. When you select the Shape, drag the yellow square 'handle' to the right or left to increase or decrease the curvature.

You also may be interested in this PowerPoint 2016 Help Article:

Combine shapes to create a custom shape in PowerPoint 2016 for Mac