Randy's idea of looking at how the old-timers did it is a good one, I think. For example, I'm finishing up restoration of an 1820 Greek Revival farmhouse that is exactly as you describe - timberframed, but visible only in the basement and attic.

In this case, the posts were 6x10, and the side walls are 6" thick and the gable-end walls 4". (Of course the insulation of choice at the time was brick, or nothing). Side wall studs were added between the posts to hold siding and lathe. Some were tenoned top and bottom, some just nailed in place. Many side wall studs were on the flat, and doubled with a gap between (i.e. separate studs for exterior siding and interior lathe). This has the advantage in modern days as allowing a little insulation between and creating a thermal break. Some studs are required for window framing. They were quite liberal in just nailing scraps wherever needed to create a landing for lathe.

Overhead, second floor and attic joists are sawn 4x8's spaced close enough to nail the lathe to, no additional nailers needed.

If it were me, I'd just use rough-cut studs from the same supplier my timbers come from.

Hope this makes sense - happy to clarify if I can.

Cheers,
Dan