Summarize the following:
It's the icon that resembles a blue and white smiley face. It's in the Dock at the bottom of the screen. Use the Finder to open the folder containing the photo you want to resize. You can use the sidebar on the left to open common folders on your Mac. Preview is the default image viewing app on Mac. You can normally open an image in Preview by double-clicking it. If Preview is not your default image viewer, use the following steps to open the image in Preview.   Right-click on the image. If you are using a Magic Mouse or trackpad, click with two fingers. Click File. Click Open With. Click Preview.app. It's in the menu bar at the top of the screen. You'll find this in the Tools drop-down menu. Use the drop-down menu next to the "Height" and "Width" boxes to select a percentage. The default unit is "Percentage". You can also select "pixels". "inches", "cm", and more. You can use either one of these boxes to resize the image. If you selected "Percentage" type what percent you want the new image size to be. If you selected "pixels", or "inches" or another unit, type however many units you want to new image size to be in the boxes.  Ensure that the checkbox next to "Scale proportionally" is checked so that the image is not distorted when you resize it. Alternatively, you can click the drop-down menu next to "Fit into" and select an image size to quickly resize the image. It's in the lower-right corner of the "Image Dimensions" window. This applies your changes to the photo. It's in the menu bar in the upper-right corner. It's in the File drop-down menu. This will save your file on your computer using the dimensions that you selected. To save the image in a different format, click Export… in the File drop-down menu, then click the "Format" drop-down menu and select one of the following image formats:   JPEG - best for photos on the web. Compact files.  JPEG-2000 - high quality, good compression. Smaller files.  OpenEXR - best used to compress video files.  PNG - best for graphics and small Web files. Larger files.  TIFF - best for editing and storing images. Larger files.

Summary:
Open the Finder . Navigate to the photo you want to resize. Open the image you want to resize in Preview. Click Tools. Click Adjust Size…. Select a unit of measurement. Type a new number in the "Width" or "Height" boxes. Click OK. Click File. Click Save.