The size of the avatar, expressed as the radius (half the diameter).
If radius is specified, then neither minRadius nor maxRadius may be
specified. Specifying radius is equivalent to specifying a minRadius
and maxRadius, both with the value of radius.
If neither minRadius nor maxRadius are specified, defaults to 20
logical pixels. This is the appropriate size for use with
Changes to the radius are animated (including changing from an explicit
radius to a minRadius/maxRadius pair or vice versa).
final double radius