I'm suggesting 4-8 hours for a single play through.
A short running time will enable use to have a large roster of characters and pack it full of novel situations which vary based on who is in your party without it becoming overly daunting.
The game should use the Witcher 2 nonlinearity model. It begins in the same place, branches off based on your decisions for the middle portion, and converges at the end.
Rather than a sprawling epic, I think we should make a small gem with many different facets.