The measuring tool is located in the toolbox as indicated. It works similarly to the Photoshop tool in the tutorial.
When using the rectangle select, you don’t have to use the free select. Use the fixed size option as shown here (after doing the calculations in the tutorial).
Make sure to specify the new file in inches according to how you want to print it. Then the resolution should be made to equal your DPI (this makes sure that your picture will turn out to be the right size).
Also, not to sound like a fanatic but long live open source software! :D
Update: if you specifically want a U.S. passport picture, the State Department has this Adobe Flash tool, which is extremely easy to use!