Use Edit -> Resize to resize the canvas, including all preceding content layers. For example, this layer can reduce the dimensions of a high resolution image to make it small enough to send via email.
Units - Use the units pulldown list to change what the width and height fields represent. You can resize by pixels, percentage, inches, or millimeters .
Pixels per Inch/Millimeter - This option is only available when the units are set to inches or millimeters. It's used to convert the set number of inches/millimeters to the image's dimensions in pixels.
Width - Use this to adjust the width of the image. The value represents the width of the image in the specified units above.
Height - Use this to adjust the height of the image. The value represents the height of the image in the specified units above.
Keep Aspect Ratio - Use this checkbox to keep the image's original proportions. Uncheck this option if you want to stretch or squash the image.