You can add space to one or all sides of an image. Added canvas appears in the currently selected background color on the Background layer; in other layers, the added canvas is transparent. In the new canvas space, you can add text or resize your photo to fit the space.
In the Width and Height boxes, enter the full dimensions of the new canvas. Choose the units of measurement you want from the adjacent menus. The Columns option measures width in terms of the columns specified in the Units & Rulers preferences, and only applies to size reductions, not size increases.
Select Relative, and enter the amount by which you want to increase or decrease the size of the canvas. Enter a negative number to decrease the size of the canvas. Use this option if you want to increase the canvas by a specified amount, such as 2 inches on each side.