Change Canvas Size

In full screen, click on edit icon in Control Bar, select Canvas size in the popup menu. A "Canvas size" dialog will show up.

Please note that changing canvas size is totally different from resizing image size. It just changes the width and height of the image without enlarging or shrinking the image content. So if the new size is larger than the original one, a background color will be filled in the blank areas. If the new size is smaller, the image will be trimmed along the borders.