Dan:
Each timber frame design can be different. Some frames sit on regular pressure treated sills with no timbers sills.
Some frames have conventional stick build first floor decks, with cut outs for the post to go threw the decking to the PT sill below, and therefore no timber sill.
It all depends on the actual design of the frame and if the first floor is going to be conventional stick built or timber framed.
If it's going to be timber framed then the sizing of the sill could depend on the floor joist connections.
In Jack Sobon's book "Build a Classic Timber-Framed House" he used a timber floor system for the first floor and he used 8"x 9" sills, that is 8" tall and 9" wide. These sills allowed enough space to create the pockets for the floor joist and other timbers to join them. His design was so that the exterior siding planks could be nailed to these sills and hang down a bit beyond the level of the sill covering the top of the foundation wall to shed the weather.
As mentioned each design is different and you'll need to size your sills accordingly, or don't use sills at all, depending on your specific needs.
Good luck with your project.
Jim Rogers
