Photoshop gives a few measurement units for setting your own dimension, namely- Pixels, Inches, Centimeters, Millimeters, Points, and Picas. Here is the description of the following units of measurement scale.
The smallest unit of measurement in an image or graphic displayed on a digital device is a pixel or picture element. Pixels (px) combined together make an image, artwork, video, and text. Converting 1 pixel to millimeter, you get, 1 px = 0.352777778 mm whereas, 100 pixels equal to 1.0416666666666667 inches in 96 dots-per-inch (DPI).
Choose centimeters (cm) if you want your document dimensions in centimeters. When converting 1 centimeter to pixels in 72 DPI you get 28 pixels. For example, if your document size is 10 cms X 20 cms in 72 DPI resolution you get 283 px X 567 px document size.
A millimeter (mm) is another unit measurement given in Photoshop to create your custom document size. It isone-thousandth
Picas (pc) contains 12 points and is less than 1/6 of an inch or 1/72 of a foot. You can picas use to represent fixed horizontal measurements and column width. Newspapers, ads, magazines, journals, and newsletters designing are done in picas. 1 picas are equal to 16 pixels. Picas, points, and other units are used for design.