Constant rounded rectangle corner radius in Illustrator

Use Effect>Stylize>Round Corners…

Create a square-cornered rectangle and apply the rounded corner effect with your desired radius. This will keep the corner radius at a constant value, however you resize the object or line.

This avoids the annoying problem of “squashed” radii on rounded rectangles or paths after resizing.

Or, if you like using rounded rectangles, you can adjust the size of the corner radius while drawing.