What foundation is best for hex structure?

[rickanova72]

  Need some advice on a foundation for a hexagonal structure i'm building using pt 6" posts and poured concrete footngs. Plan on building 2 stories above this all with wood. It is in Ocala where it's mainly sandy soil. It will be built around an existing hex structure that is post built and then tied into the new structure.
  My question is how large should the holes be in depth and width? The size is 12 ft.from corner to corner forthe new building .

[Squirl]

My understanding from a quick search is that Ocala is in central Florida.

IMHO, I wouldn't build anything in that area without high wind design.  Here is the guide to the American Wood Councils Special Design Provisions for Wind and Seismic standard with Commentary.
http://www.awc.org/pdf/2008WindSeismic.pdf

Florida codes are the ICC, with some special provisions.  The ones I am familiar with is that for 2000 psi soil (sandy) and two stories the footing has to be 12" wide and 7" thick. (2x8)
http://publicecodes.cyberregs.com/st/fl/st/b400v10/st_fl_st_b400v10_4_sec003.htm
From a quick read, Florida seems to have a requirement of connecting the footing to the foundation with rebar.  This may be because of the high wind standards.

From recollection the ICC requires that footings must be 12" deep when there is no frost line.

So when you say corner to corner, do you mean 12 foot diameter or per side of the hexagon?