Use this function to set a picture’s size in pixels. You can also use it to set a physical size if you first tell the program a desired number of dots per inch (DPI) for the picture.
To resize a picture, use Edit | Resize… [Ctrl+E].
Resize Layer [Ctrl+SHIFT+Alt+E] – use this Editor option to resize individual layers.
To set the Width and Height to resize to, enter them in the box. Normally they are set in pixels, but if needed, use the controls to set them in percent, centimeters, millimeters, or inches instead. Digital pictures’ actual dimensions are always in pixels, so sizes in centimeters, millimeters, or inches are relative to a dots per inch (DPI) value. Use the DPI control to set this value. It tells printers, etc. how many pixels should be used per inch during printing, display, etc. The appropriate DPI setting to use depends on how the resized picture will be used. For screen display, 96 DPI is fine. For print, at least 150 is recommended. The best choice of DPI also depends on specifics like printer quality, paper quality, etc.
Leave the Keep proportions option on to make the program automatically set one dimension (width/height) based on your value for the other one, so that the picture isn’t distorted. To enter Width and Height independently, turn this option off. Sharpen reduces the harm caused by the loss of information involved in shrinking a picture. (Major shrinking can cause a loss of detail, and mild sharpening can lessen this problem.) To set the method used for resizing, use Method.
If multiple pictures are selected, you can restrict the function so it only shrinks or only stretches pictures, using the control named Mode.
Save image only saves disk space by stripping away any EXIF or other picture information that may be in the resized pictures.
More information