I use an 8x10 tractor. Only because that is the wood I had around, so that is the size I made it.
Bigredfeather is on the money with 15lbs to about 8 weeks.
I agree with trying to split a bulk order of feed. I was only able to do this once so far, seems I can never find a partner to split with when I need feed. I must admit that I usually end up buying a bag of chick starter, because I am a sucker, and end up getting chicks before I get to the mill. Once we get rolling, there is enough turn around to keep the feed fresh. I give some too the layers if I am not using it up fast enough.