To adjust the size of the canvas follow Image > Canvas Size or press the shortcut key Alt+Ctrl+C. This will appears the Canvas Size dialog box.
There you are able to adjust the Width and Height of the canvas using the selected unit. If you need to keep the aspect ratio of the canvas, then you have to select Relative check box. Use the Anchor position to show where to begin resizing and what direction. You are also able to select the color of the extension by using the combo box named, Canvas extension color. Press OK to save the settings to the canvas.
(Click Cancel button to cancel settings and close the dialog box. If you need to reset the settings to the beginning position which is at the opening of the dialog box without closing the dialog box, press <Alt> key + Click on the Reset button. The cancel button change to Reset when you press the <Alt> key.)