The size and color of the suffix icon is configured automatically using an IconTheme and therefore does not need to be explicitly given in the icon widget.
The suffix icon is constrained with a minimum size of 48px by 48px, but can be expanded beyond that. Anything larger than 24px will require additional padding to ensure it matches the material spec of 12px padding between the right edge of the input and trailing edge of the prefix icon. The following snippet shows how to pad the trailing edge of the suffix icon:
suffixIcon: Padding( padding: const EdgeInsetsDirectional.only(end: 12.0), child: myIcon, // myIcon is a 48px-wide widget. )
The decoration's container is the area which is filled if filled is true and bordered per the border. It's the area adjacent to decoration.icon and above the widgets that contain helperText, errorText, and counterText.
final Widget suffixIcon