Controls the size of this button.
By default, floating action buttons are non-mini and have a height and width of 56.0 logical pixels. Mini floating action buttons have a height and width of 40.0 logical pixels with a layout width and height of 48.0 logical pixels. (The extra 4 pixels of padding on each side are added as a result of the floating action button having MaterialTapTargetSize.padded set on the underlying RawMaterialButton.materialTapTargetSize.)
final bool mini