static class CanvasExtensions {
public static Vector2 SizeToParent(this RawImage image, float padding = 0) {
    var parent = image.transform.parent.GetComponentInParent<RectTransform>();
    var imageTransform = image.GetComponent<RectTransform>();
    if (!parent) { return imageTransform.sizeDelta; } //if we don't have a parent, just return our current width;
    padding = 1 - padding;
    float w = 0, h = 0;
    float ratio = image.texture.width / (float)image.texture.height;
    var bounds = new Rect(0, 0, parent.rect.width, parent.rect.height);
    if (Mathf.RoundToInt(imageTransform.eulerAngles.z) % 180 == 90) {
          //Invert the bounds if the image is rotated
          bounds.size = new Vector2(bounds.height, bounds.width);
    }
    //Size by height first
    h = bounds.height * padding;
    w = h * ratio;
    if (w > bounds.width * padding) { //If it doesn't fit, fallback to width;
        w = bounds.width * padding;
        h = w / ratio;
    }
    imageTransform.sizeDelta = new Vector2(w, h);
    return imageTransform.sizeDelta;
}
}