> a human-readable size—18k loc or so.

I agree in general with the principle that there is a 'human-readable' size, and that it's around some 10k-30k LOC, but I'm curious how you came up with that specific number. Has it been researched and measured? Does it vary significantly from person to person? I tend to think of 80kLOC as the upper limit, because that's the size of the DOOM engine, written by a single highly-capable programmer, who presumably had it all in his head at one point. But that is empirically way beyond what I'm capable of.