Do colleges expect that students add information here, or is it ok to leave blank if everything has been covered in the rest of the Common Application? If you are to add information, how long should it be and what should it include?
No, it is not expected. Do not add extra essays, for example. Things that might make sense would be if you have more awards or activities than fit on the app. Or one of my kids did MOOCs that do not fit in any other category on the app. My kid’s school has a funky way of naming one of their honors math sequence classes that doesn’t have the word “Honors” in the class title (ugh… don’t ask), so my kid put in a one sentence explanation about that.
I put some stuff there that I thought would be useful, but it wasn’t very much. I want to be an engineer and luckily had been able to use CAD software through Boy Scouts. It was like 2 sentences, but there was nowhere else to put it.