code: public void setLayerSize(int index, int w, int h) {
          final ChildDrawable childDrawable = mLayerState.mChildren[index];
          childDrawable.mWidth = w;
          childDrawable.mHeight = h;
      }
docs: Sets an explicit size for the specified layer.
  <p/>
  <strong>Note:</strong> Setting an explicit layer size changes the
  default layer gravity behavior. See {@link #setLayerGravity(int, int)} for more information.
  
  @param index the index of the layer to adjust
  @param w     width in pixels, or -1 to use the intrinsic width
  @param h     height in pixels, or -1 to use the intrinsic height
  @attr ref android.R.styleable#LayerDrawableItem_width
  @attr ref android.R.styleable#LayerDrawableItem_height
  @see #getLayerWidth(int)
  @see #getLayerHeight(int)