As Size, Width and Height are Get() properties of System.Drawing.Image;
How can I resize an Image object
If you're working with a BitmapSource:
var resizedBitmap = new TransformedBitmap(
bitmapSource,
new ScaleTransform(scaleX, scaleY));
If you want finer control over quality, run this first:
RenderOptions.SetBitmapScalingMode(
bitmapSource,
BitmapScalingMode.HighQuality);
(Default is BitmapScalingMode.Linear which is equivalent to BitmapScalingMode.LowQuality.)