The size and color of the prefix icon is configured automatically using an IconTheme and therefore does not need to be explicitly given in the icon widget.
The prefix 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 left edge of the input and leading edge of the prefix icon. The following snippet shows how to pad the leading edge of the prefix icon:
prefixIcon: Padding( padding: const EdgeInsetsDirectional.only(start: 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 prefixIcon