Pixels Per Unit: Images Tab

If the image file loaded is a different Size than the last file loaded, it will ask you to set the Scale using Pixels Per Unit (the current Unit of Measurement) for the Image.

If you are using Inches, it will be Pixels per Inch.

Enter the number of Pixels Per Inch and click OK.

Optionally, click the Set as Default checkbox to set this size as the default size.