Im building a pole barn with loft for storage 40'X40'X12' for live stock,trucks, and cars. I want to ancore the posts to the concrete. How thick does the concrete need to be and how thick do the footers need to be?

Location of this building has a lot to do with the post settings and depths,etc.

If you are in some states where the winds are high at times,the depth and size of posts vary depending on the needs.

Anchoring the posts to the surface isn't a desired method for a building of this size,high winds/tornadic activity can simply lift and remove the entire structure.. I would suggest burying the posts as deep as possible(most builders of poll barns have specifics of what they use in their areas,much less codes which vary from state to state.)

Typical footers are below the frost line for your region,concrete(I am guessing you are describing the curbing around the perimeter)is whatever you feel would suffice for the needs, cattle require scraping and with the amount of force a loader can apply,the curbing would need to be able to withstand a direct hit from a moving loader.

In most states a pole barn of this size will require building pormits and stamped architectural drawings to get those permits.

Those plans will also provide footing, foundation and anchoring information.

If you don't have such plans, you really need to get them, because there is no other way to determine the loads and the anchoring that needs to go into such a structure.

engineered plans are ideal, but if your handy? And determined get at least 16 foot posts and auger 4+ foot holes, You'll need a power auger about 4in. Bigger diam. Than posts,concrete, but if your really old school,put a big round cobble at bottom of hole set post plumb with a couple of 2x4s and put 4inch layers of sand then crushed rock and tamp each layer,this is the best way but more work. You should probably creosol tar bottom four foot of post. As for exterior wall at least 18in x 18in footings remember to put your anchor bolts, L bolts when pouring concrete. To bolt your pressure treated 2x6 bottom plate. Build the walls on the slab and tilt them up in place.bunch of guys for tilting up wall or back hoe. Determination and some good common sense, In my opinion anybody can do just about anything. One thing, 12 feet high with a loft, I prefer a loft you can stand in. You would need taller poles or posts like 20 footers, sink them at least 5 feet. Have fun !

All good advice... but... I have built many... I use 6 x 6 treated posts... bore 4 ft deep hole about 6" larger in diameter than the posts... stand the post in the hole... hold post plumb in both directions... pour a gallon of water in the bottom of the hole... dump 'sack-crete' around the post in layers... tamping with shovel handle as you go... maintain plumb both directions... stop the sack-crete a few inches from the top of the hole... top off with water... the water at the bottom & top of the hole will migrate to harden 'all' the sack-crete by next day...

NOTES !

1. This method assumes you are working in undisturbed soil ! I f you're in fill dirt... move the barn or consult an engineer.
2. SET THE 4 CORNERS FIRST ! Then pull a string (about chest high) to get the posts in the middle in line. The tops of the posts will be limber enough to pull into line as you build the roof and set trusses. (I hope you're planning to use trusses !) Also... use at least a doubled 2x 8 top band with posts no farther apart than 6 ft.
3. This also assumes your local codes don't prohibit an old farmer building his own stuff his own way.