I've been doing some hunting trying to find answers to many of these questions. Here is what I've found.
Most milksnakes that you are going to find in the pet trade are not pure. There are 25 or so of the Lampropeltis triangulum. The divisions after that are subspecies. So your Nelson's is probably a different locality with slightly different characteristics than say a Sinaloan, Pueblan, or Honduran. There are some general trends it seems, but because of the high degree of crossing between subspecies, its really hard to pin down a for sure length, behavior, width of bands, etc.
Nelson's if you can find a pure type, do tend to be a bit smaller. Many of these are actually Sinaloan x Nelson hybrids.
Your Honduran milk snakes will be longer, but again its hard to say how much longer. Many of these are crossed with L.t. polyzona. It seems as though Hondurans are capable of being one of the larger types.
A Sinaloan or Pueblan will be in the middle between Nelson's and Hondos.
I don't know much about behavioral traits. I've generally heard that proper handling over time can reduce their squirming. Milk snakes to tend to be flighty, as are kings.
Here are some links to info, and a link to a thread over at kingsnake.com that has a photo of a giant Hondo. I would ask those guys, they seem to know a lot.
http://www.kingsnake.com/king/ssp.html
http://www.pitt.edu/~mcs2/herp/Lampropeltis.html
http://www.herpnation.com/hn-blog/th...-milksnakes-2/
http://forums.kingsnake.com/view.php?id=1975212,1975234