The size of the icon inside the button.
If null, uses IconThemeData.size. If it is also null, the default size is 24.0.
The size given here is passed down to the widget in the icon property via an IconTheme. Setting the size here instead of in, for example, the Icon.size property allows the IconButton to size the splash area to fit the Icon. If you were to set the size of the Icon using Icon.size instead, then the IconButton would default to 24.0 and then the Icon itself would likely get clipped.
final double? iconSize;