The short, and flip, answer is as long as it needs to be, but no longer and as short as you can make it, but no shorter. However, I like to keep it to a page so that it is both usable and lean.
I recently read Checklist Manifesto by Atul Gawande. I didn’t expect to learn much because we’ve been doing this for years, but I was surprised at how good Dr. Gawande’s treatment was. He told a number of great stories and presented that provided insights I had never considered and did some very good historical research. I’ll save these for another time. Today, I want to nitpick on a bit of wrongheaded advice about the size of a checklist.
A rule of thumb some use is to keep it to between five and nine items, which is the limit of working memory. Boorman didn’t think one had to be religious on… (page 123)
Really? I’d read the original source, The Magical Number Seven, Plus or Minus Two: Some Limits on Our Capacity for Processing Information, Psychological Review 63 (2): 343–355. http://psychclassics.yorku.ca/Miller/. http://www.psych.utoronto.ca/users/peterson/psy430s2001/Miller%20GA%20Magical%20Seven%20Psych%20Review%201955.pdf and it had seemed to me a good reason to make a checklist, not a limit on that checklist.
So I did some research and found out Miller had addressed this issue directly in a correspondence with Mark Halpern. (http://members.shaw.ca/philip.sharman/miller.txt) Halpern wrote to Miller seeking help with what he considered to be a misreading of Miller’s original paper
“The director of the technical writing group which I serve as editor has issued an edict that lists and procedures in our printed and screen-displayed documentation should not exceed seven, or maybe nine at the most..”
Miller wrote back,
“the point was that 7 was a limit for the discrimination of unidimensional stimuli (pitches, loudness, brightness, etc.) and also a limit for immediate recall, neither of which has anything to do with a person's capacity to comprehend printed text”
I also found this story with additional commentary on documentation guru Edward Tufte’s web site. http://www.edwardtufte.com/bboard/q-and-a-fetch-msg?msg_id=0000U6 . It’s interesting reading.
There are other good reasons to keep the checklist short, but the number 7 research is relevant neither to document design nor to how people comprehend printed text.