Images in Acorn can be in units of pixels, inches, or centimeters. When creating a new image in Acorn, you will be given the choice of which measurement unit you wish to apply.
If you wish to change the measurement unit in an existing image, you have a couple of options. You can go to Acorn ▸ Preferences ▸ General ▸ Ruler Units and make your selection.
Or you can modify the base measurement unit if you have rulers present on your image (View ▸ Show Rulers). Right click on one of the rulers and make your selection.