Thickness of pier for small cabin

[lesliegray]

Hello,
I'm planning to build a small cabin measuring 16'x40' and will be using concrete piers for support. I intend to run three 4x6" girders along the side walls and center, with piers placed every 8 feet. The total load is estimated at 50 PSF, with a soil rating of 1500 PSF and a tributary load of 3200 PSF (64 sq ft).

I'll pour footings with two cinder blocks high to support and attach the girders. My plan is to create 24"x24"x6" footings with #4 rebar placed horizontally. While I've calculated a conservative footing size of 18"x18"x12", I recognize that the girders will help distribute the weight.

Would the 6" thick footings with rebar be sufficient, or what is the minimum thickness required for the poured footings? I would appreciate some advice on this matter.