In the Pixel Editor's Prop Definition window, use a predefined shape if there is one there that meets your requirements. Any shape can be resized and rotated, so you are not limited to the traditional orientation of the shape. Consider using a horizontal matrix for net lights (bushes, tree wraps, etc.).
The selections in the middle will change depending on the shape that is selected and type of lights (traditional, RGB pixels, or dumb RGB). At the bottom will be diagram of your shape primarily intended to show channel number orientation for props with multiple channels.