Engineering specs for roundwood timbers

i am playing around working on framing ideas for our next, and final, house. We want to use roundwood that we will harvest from the site to build this, but as I am sketching along and working up the numbers of beams and posts and rafters, it occurred to me that I have no data on what Size a given post, beam, or rafter needs to be.
Anyone know of a source that can tell me, for example, what diameter of red oak timber is needed to bridge an eight foot span? If the total roof area is going to be 40x70 feet, with a single pitch, where can I find formulae for calculating the load on my posts, and then guides for what size they need to be for that load?

Am I even asking these questions in a way at makes sense?

Any help would be appreciated.

Yes, your question makes perfect sense.
What you are trying to do is calculate the load bearing capacity. I'm still a bit in the dark myself, but I was recommended Jack Sobon's "Build a Classic Timber Framed House" as there is supposed to be a pretty good section in the back on the math for engineering a beam. I've got it on order at my local library, I'd suggest you do the same. If you're not interested in doing that, then all I could suggest would be finding a architectural engineer and either hiring him, or plying him with dinner and a six pack to walk you through the math to do it.