how can I enlarge or shorten a rectangle?

I use (and teach) Silkrooster​'s technique—Free Transform with Ctrl/Cmd T. But here's an alternative that some of my students prefer—turn on Show Transform Controls, which allows you to directly edit the object without having to select Free Transform.

Make sure the correct layer is selected in the layers panel. Then use the short cut ctrl-t on Windows or cmd-t on Mac. When done scaling, press enter/return to commit.