Solo,
In the contruction book it says that a .61 2-stoke gives very good performance. It shows a .61 in the pictures as well as an .82 4-stoke in the build booklet. Although some guys opt for the 20cc I would think balancing it would be a problem, as my 260 balances perfect with the .81a 4-stroke. Of the ARF's of the Extra that I have built, I rate the Aeroworks one as having the best build quality.
Karl