[Cacti]

One page of text at standard size, with standard style (one statement or declaration per line). Which is roughly 60 lines.

If you follow their other guidelines (no recursion, fixed sized loops, etc.), then it can take you several lines to do much of anything, so 60 isn't very long. Those things combined means you're basically saying that each function should only be attempting to do 2-3 significant things, which seems about right to me, as most people can't fully understand more than 3 things simultaneously at one go.