Open the resize-practice-small.jpg from our last exercise and choose the Canvas Size command so you can look at the dialog box now. Canvas Size can be accessed from the title bar context-sensitive menu in Windows or from the Image menu in Windows or Mac.
As you can see, the current size is shown at the top. There are fields for entering new width and height dimensions. You can enter your dimensions as percentages, pixels, inches, cm, mm, points, picas, or columns. The anchor diagram lets you specify how the canvas is modified. By default, the middle box is selected which modifies all sides of the image equally. When you select any of the other squares, the canvas is modified in the opposite direction. For example, pressing the bottom middle square will add or subtract from the top of the canvas.
The "Relative" check box determines whether the resizing is absolute or relative to the existing canvas dimensions. If you know the new width and height you want for the total image size, you would leave the relative box unchecked and enter the new sizes you want. However, if you just want to add or subtract a certain amount of space around the image, you can check the relative box and enter the increase/decrease amount to avoid doing the math.
The Canvas Extension Color menu allows you to choose what color is used to fill the new space being added to the borders of your canvas. By default, the new canvas area is added using the current background color, but you can use the menu to choose foreground, background, white, black, gray or other. When you choose other, a color picker opens, allowing you to select a color. You can also skip the menu and choose any color by clicking the small color square to the right of the menu. While the color picker is open, you can move your cursor outside of the color picker and over your document. The cursor will change to an eyedropper and you can click to select any color from any open document in Photoshop. If your document does not contain a background layer, the Canvas Extension Color menu is is unavailable and the new canvas area will be transparent.