[eriqjaffe]

And if you don't have Photoshop, you can do it with GIMP!

1. You'll need to unzip the attached python script into your plug-ins folder. The default path for Windows is C:\Users\(your name)\AppData\Roaming\GIMP\2.10\plug-ins. You can find the path by going to Edit -> Preferences -> Folders -> Plug-ins. Note that if you are using macOS or Linux you'll need to make sure the script is executable:
   

   chmod +x selection_dimensions.py

2. Create the mockup, same basic instructions as the previous post.
   
   
3. Use the magic wand tool to select a rectangle.
   
   
   
4. From the "Select" menu, choose "Show selection margins"
   
   
   
5. The coordinates will show up in a dialog box.