The default font is mono-spaced, so there is no width data required - it's all the same: 16. It's really not a good example for the spacing feature.
well, it's supposed to be, but it's not really. Capital 'I', 'T' and many others are not correctly aligned. I guess it's just a not-so-great font sprite.
I'm teaching a class, and want to start them with SpriteFont instead of the Text object, but that's proving difficult since the default sprite font is not very aesthetically pleasing. And while the GiveYourFontsMonoSpace program is excellent, the workflow is a little too cumbersome for my students.