You can perform this by choosing the Leader Style for the Dimension. To do So:
1.Draw a Arc Or Circle
2.Go for Smart Dimension and select the Arc or circle.
3.Then Go to Leaders option and select the Desired One. FYR,I have attached the screenshot.
Thanks & Regards ,
1.PNG 19.4 KB
You can right click on a Radius dimension & scroll down to choose "Display as Diameter". If it's not visible you might need to "Customize" your menu & add it.
Not sure if there is a way to always show as a diameter by default. Somone on the Forum may be able to write a Macro to do this.