1. override
GridSpecification getGridSpecification(BoxConstraints constraints, int childCount)

Override this method to control size of the columns and rows.

Source 

@override
GridSpecification getGridSpecification(BoxConstraints constraints, int childCount) {
  assert(constraints.maxWidth < double.INFINITY);
  final int rowCount = (childCount / columnCount).ceil();
  final double interiorWidth = constraints.maxWidth - padding.horizontal;
  final double columnWidth = interiorWidth / columnCount;
  final double tileWidth = math.max(0.0, columnWidth - columnSpacing);
  final double tileHeight = tileWidth / tileAspectRatio;
  return new GridSpecification.fromRegularTiles(
    tileWidth: tileWidth,
    tileHeight: tileHeight,
    columnCount: columnCount,
    rowCount: rowCount,
    columnSpacing: columnSpacing,
    rowSpacing: rowSpacing,
    padding: padding
  );
}