Most of your images will not be sized to the exact dimensions you need. You have several options at your disposal.
To change the size of an image in Photoshop, you can use the Image Size or Canvas Size command. You can also use the Crop tool or Free Transform command to make an adjustment. You can use these choices individually or in combination to achieve the desired results.
The Image Size command lets you permanently reassign the total pixel count, as well as resolution, for a particular image. You can also use this command to upsample or downsample an image. This is an easy way to size an image to a specific height or width.
With your image open, choose Image > Image Size, or press Cmd+Opt+I (Ctrl+Alt+I on Windows). The Image Size dialog box offers several choices. You can choose to manipulate the pixel dimensions of the image (measured in pixels or percent).
You can also modify the print size, which is the size of the image when printed. You can modify the print size based on percent, inches, centimeters, millimeters, points, picas or columns. The most common choices are percent, inches, or centimeters, because most users easily understand these units of measure.
Then, set the Document Size to measure in inches. Be sure to select the Resample Image option if you want to change the pixel dimensions. Choose the method to Resample Image that is most appropriate for your image. Bicubic Automatic is the most common method, but you may have special circumstances. Click here for more information.
Leave the Constrain Proportions check box selected, or you will introduce distortion. You generally want to keep the width and height constrained to the same ratio so the image resembles its original appearance.
Finally, enter a resolution value and click OK.
The canvas size is your work area. When you create a new document, you can specify the size of your canvas. When you scan a photo or import a digital image, the canvas size is automatically set to the edge of the image. You may need to change the canvas size to crop or extend the canvas of your image to work on specific areas of the image.
To get started open your image and choose Image > Canvas Size, or press Cmd+Opt+C (Ctrl+Alt+C on Windows). In the Canvas Size dialog box you’ll see the dimensions of your current canvas. You can specify a new canvas size using a variety of measurements. Pixels is a useful measurement if you’re creating screen graphics, whereas inches or centimeters is easier to understand for print work. Using percentage is also good for incremental amounts.
Be sure to select the Relative check box. This disregards the numerical values of the current canvas size and allows you to specify a new amount to be added to the existing image.
Ensure that the anchor point for the image is set to centered. This will expand the border in all directions around the center of the current image.
Finally, specify a Canvas extension color. This is the color that Photoshop places around the image when you change the canvas size. You can choose to use the foreground or background colors that are loaded in the toolbox. You can also use white, black, gray or other, which can be any color you specify. Click OK.