I am having a hard time figuring out a post and concrete footing syste

I am having a hard time figuring out a post and concrete footing system to support a 12 ft long x10 ft wide shed on a slanted grade. If my calculation are correct the shed will weigh about 3000 lbs. I have clay soil. The front portion of the shed will be very close to the ground, but due to the grade of my property, the rear will be in the area of 3ft off the ground. I plan to dig about 48" deep and place 8 inch round concrete footings. I plan to use 2x8 treated lumber for my floor joist placed 16" O.C., 3/4 inch plywood for a floor. What size should my post be (4x4,4x6 or 6x6)? and can you recommend a pattern for placement of the post. I will be keeping a riding lawn mower in the shed. Please help me.