The MaxRadius property returns the maximum radius value possible for a rectangle's rounded corner. This value is half the size of the shortest side of a rectangle.
Examples:
The following VBA example sets the maximum corner radius for all rectangles to 0.5".
Sub Test()
Dim s As Shape
Const r As Double = 0.5
For Each s In ActivePage.FindShapes(Type:=cdrRectangleShape)
If s.Rectangle.MaxRadius >= r Then
s.Rectangle.SetRadius r
End If
Next s
End Sub