Dfalpha,
I believe this Spit has wingspan in the 52 - 60 inch range. I recall they were built very light with a weird foam like covering. I believe a .61 two stroke will fly it with plenty of authority. I would not go much lower than a .46 two stroke. I would recommend contacting GP if you have any other questions, ie.: CG.
"Keep 'Em Flying!"
Flak