Both your and DLB's reply has got me thinking that maybe I have asked the wrong question and instead I should have asked for the best way to create / shape a cruck blade since this might be more easily undertaken with the blade lying "flat" on the ground.
Small cruck blades would be 16" or so diameter at the butt tapering to 12" dia or so at the top end and would probably be about one rod long (16 - 17ft). Where the blades need to rise to peak at the ridge then they might need to be up to 28 - 30ft long with varying degree of elbowing.
I came across a picture today of The Barley Mow pub at Long Wittenham. This is just down the hill from the Wittenham Clumps project that was undertaken by the TFG a few years ago alongside the Carpenters Fellowship and this photo shows the exposed cruck blades of the timber frame following a fire in the thatched roof during the 70's.
The Barley Mow cruck blades are made from Wytch Elm and currently I have a number of standing dead elm snags at the woodlot which have recently succumbed to Dutch Elm disease. Elm is quite a stringy wood to work having more than its fair share of both cross and wild grain.