In the motion tap the Crop menu is located directly below the Basic Motion menu. Click on it, which will then open to reveal options to cut off some of the top, bottom, sides, and edge feather for making soft edges. You then drag the bars over for each side to the point that you would like them to be on the image. Do this while watching the image in the Canvas because you will see how each move of the bar in the Crop tab will affect the image. With this you can put black bars at the top and bottom of the image, or try and do something unique like make the image a perfect square.