Most of your images will not be sized to the exact dimensions you need. You have several options available to you.
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 the 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 allows you to permanently reassign the total number of pixels, as well as the resolution, for a particular image. You can also use this command to upsample or downsample an image. It’s an easy way to resize 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 offers several choices. You can choose to manipulate the pixel dimensions of the image (measured in pixels or percentage).
You can also change the print size, which is the image size when printed. You can change the print size in percentage, inches, centimeters, millimeters, points, picas, or columns. The most common choices are percent, inches, or centimeters because most users easily understand these units of measurement.
Next, set the document size to a measurement in inches. Be sure to select the Resample image option if you want to change the pixel dimensions. Choose the image resampling method that best suits 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 the box or you will introduce distortion. You generally want to keep the constrained width and height at the same ratio so that the image looks like it originally did.
Finally, enter a resolution value and click OKAY.
The size of the canvas is your working 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 your image canvas 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, you will see the dimensions of your current canvas. You can specify a new canvas size using a variety of measurements. Pixels are a useful measurement if you’re creating on-screen graphics, while inches or centimeters are easier to understand for print jobs. Using a percentage is also useful for incremental amounts.
Be sure to select the Relative check box. This ignores the numeric values of the current canvas size and allows you to specify a new amount to add to the existing image.
Make sure the image anchor point is centered. This will expand the border in all directions around the center of the current frame.
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 loaded in the toolbox. You can also use white, black, gray or whatever, which can be any color you specify. Click on OKAY.