1. 简单的一根直线，在注释中说明代表多长。

2. 简单的一根直线，在旁边（上面或者右边）中说明代表多长。

1. Set units of measure to pixels instead of inches or centimeters in Photoshop

Edit > Preferences > Units and Rulers. Change units/rulers to pixels. You can leave types to points or pixels which doesn't matter. 

If it already shows distance in pixels, you don't need to change it.

2. Open the info window that can show the distance

Windows>info  Now you can see the distance in pixels in the info window now.

3. Use the line tool to draw the scale bar

line tool>set the width to appropriate width (such as 5 pixels),draw the line to appropriate length based on your calculation (if you want to show 10nm, and 100 pixels represents 10 nm, then draw a straight line with the length of 100 pixel, in the info window,make sure delta Y equals 0, D equals 100. Draw the line when pressing shift can make sure delta Y equals 0.)

4. Add the legend if you want using the text tool.