From the "data so boring it wrapped all the way around to surprising" department: it turns out that in any given second, any given customer is probably either uploading or downloading or neither, but not both. Who knew, right?
Here's a plot of (hand waving badly), average downstream packet size for a given customer during a given second on the x axis, and upstream packet size on the y axis, where darkness indicates frequency of occurrence of that combination.
The black smear in the bottom right is downloads (and the tiny TCP ACK packets going the other way). The upper left is uploads. There's a dim pattern in the bottom left, along x ~= y, that's maybe pings or VoIP or something. (It's kinda interesting that the tx size seems correlated with the rx size for those packets.)
The most surprising thing for me was how uninteresting it is. What about 1000-byte packets? 576-byte packets? Sorry, but no. Alas.