In addition to sizing images by pixels or inches in Preview, you can also reduce the size of your images by a specific percentage by choosing Percent from the Dimensions list. Make sure the Scale Proportionally checkbox is checked so as to maintain proportions when resizing.
Quick Guide: Reduce the size of an image by a specific percentage in the Preview App
- Open the image in Preview.
- Choose Tools ➪ Adjust Size from the menu.
- Click the dimensions drop-down list and choose Percent.
- Enter the desired percentages in the width or height boxes.
- Ensure that the Scale Proportionally checkbox is checked.
- Click OK.