The length will probably be 8 or 9 hours for an experienced gamer.
Bethesda said the length will be from 10 to 15 hours, but I don't believe them... They said Wolfenstien will be from 15 - to 20 hours and I finished it in around 10 or 11 hours on HARD difficulty. (Died a lot, and I explored a lot).
I don't expect a very long game with this one.
As for the STEAM pre-order, I have no idea... Probably a bit early for that.