Creating Place Scale Markers
You know those scale markers they have on maps that say that 1″ equals 1 mile? Well, Photoshop can create something called Scale Markers. These are measurement guides that are created based on the measurement scale you use. Once a measurement scale is established, go under the Analysis menu and choose Place Scale Marker. In this example, I have established my measurement scale to interpret 100 pixels as 1″ in a file that's 10″ wide at 100 dpi. So if I want to create a 3″ scale marker, then I would enter 3 in the Length field. I can also choose to display text as a label for the marker. You can choose its color and placement depending on the file.