Id pick the magnet size that allows room for coils and allows you to get the most magnet on the rotors. 4" is probably a bit small (way too small actually - for a dual rotor design like this).
I think your design is interesting...
Folks often encourage others to "think outside the box" around here... which is good.
but I think it's wise first to have a very good idea about "whats in the box?" before you start changing too much. I'm of the opinion, that you can build a machine that does quite well in both low, and higher winds in a much simpler way. In most cases.. if it does nicely in low winds, you'll have more power than you need in high winds anyhow.
I'm all for experimentation and I'd love to hear the results of this interesing wind turbine, but I have a feeling - that unless you build something a bit more conventional first, you may never know if its an improvement.. or not. My guess - is the same work and effort put towards a more conventional HAWT design with a single rotor, will yield better results.
Just my thought on it all...! You could certainly build it as a conventional unit first with 1 59" blade, test that... and then move on from there!