The minimum size of the avatar, expressed as the radius (half the diameter).
Defaults to zero.
Constraint changes are animated, but size changes due to the environment itself changing are not. For example, changing the minRadius from 10 to 20 when the CircleAvatar is in an unconstrained environment will cause the avatar to animate from a 20 pixel diameter to a 40 pixel diameter. However, if the minRadius is 40 and the CircleAvatar has a parent SizedBox whose size changes instantaneously from 20 pixels to 40 pixels, the size will snap to 40 pixels instantly.
final double minRadius