TWICImage 的 Width、Height 属性, 可以读取但不能赋值. 

调整 TWICImage 的大小、比例, 可以通过 IWICBitmapScaler 接口.

//IWICBitmapScaler.Initialize 函数有四种比例变换的算法:
WICBitmapInterpolationModeNearestNeighbor = 0;
WICBitmapInterpolationModeLinear          = $1;
WICBitmapInterpolationModeCubic           = $2;
WICBitmapInterpolationModeFant            = $3;

测试代码:

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

uses Wincodec;

procedure TForm1.Button1Click(Sender: TObject);
var
  WicImg: TWICImage;
  Factory: IWICImagingFactory;
  Scaler: IWICBitmapScaler; {图像比例变换接口}
begin
  WicImg := TWICImage.Create;
  WicImg.LoadFromFile('c:\temp\test.jpg');

  Factory := WicImg.ImagingFactory;
  Factory.CreateBitmapScaler(Scaler);

  Scaler.Initialize(WicImg.Handle,
                    WicImg.Width div 2, WicImg.Height div 3,
                    WICBitmapInterpolationModeFant);

  WicImg.Handle := IWICBitmap(Scaler);
  Canvas.Draw(0, 0, WicImg);

  Scaler := nil;
  Factory := nil;
  WicImg.Free;
end;

end.