Tom: My advice to you is, let the timber frame designer decide on the sizes of your timbers. Someone who had drawn barns before will understand the sizes needed to hold up the loads you're intending to place on the timbers in your barn.
I would suggest going to the timber framers guild home page and click on resource guide and enter your state, and click leaving the category blank. This will list all timber framing companies (as well as other companies) registered with the guild from your state. There are several there. They have experience working in your state and can advise you further or help you with your frame design.
If you change the state to CT you'll find other engineers who are qualified to help you with your design and maybe able to help you get approved plans for your state.
Building into a hill shouldn't be a problem if it's designed correctly for proper support and water runoff.
You could also check with the timber framers business council for leads on engineers or timber frame designers who can help you with your plans. Their web page is: www.timberframe.org
Good luck with your project. Jim