Think carefully about what you're going to store in it, both now and in the future. You've got a lot of land and you'll find a way to fill any space you create /w3tcompact/icons/laugh.gif. You'll want plenty of headroom - that means a tall building and more room taken for stairs to get up into that loft. You won't be sorry if you take the time to sketch it all out on paper. A metal building will give you the most "bang for you buck" and by increasing the roof pitch and adding eaves, it can be aethetically pleasing and give some "upper" storage as well.
I was originally going to build something just as you described, then considered a 36x40 building, but after playing on paper with how everthing was going to fit and how to manuever inside of it, I changed the dimensions slightly to 32x48x12'. It's so much easier to have your tractor attachments on a level concrete floor. (I still haven't invested in Freedom Hitches and winter is long here). I figured it was plenty big enough, but I fillled it in no time. It now houses: Farmall M, Kubota
L3710, loader, tiller, rotary cutter, rear blade, snowblower, post hole auger, 18Hp riding mower, utility wagon for mower, DR brush mower, 8Hp snowblower, Troybuilt rototiller, small motorcycle, tool benches, tools, etc., etc. I no longer have room to store my boats in this building and have moved them to the animal barn.
You can never have too much power or too mauch storage space!
Good luck!
Pat (Techno-Tractor Mom)