When you need to cut a long and curved shape from plywood or any other material, a good tool to use is your portable circular saw. This can cut a surprisingly tight radius if you take it easy and the cut is generally less "wandering" than that which you often get from a saber-saw when trying to guide it around a very long outline.

The photo shows the results of cutting 1/4" plywood for use on my cabin side. (This is an overlay only and not the finished structural thickness of the cabin).

The tightness of the radius that you can cut depends on really two considerations; the thickness of the material and the diameter of the blade in the saw. Thin material can be cut to a tighter radius and the smaller the blade obviously, the tighter the radius which can be negotiated. A little practice will make perfect!