I like to use 2x6's on the top chords with 2x4's on all the other parts.
Squeezing it between two oak trees might be ok, but it might also be an issue down the road. I truly hate to take out an old tree, and oaks are my very favorite of all the trees. If it was me, I wouldn't build between two trees.
One problem is that you might very well kill them both with your building. I've had that happen, which really sucks. You go to allot of trouble to try and save the tree, and it dies on you anyway. Now your building is smaller then you wanted it to be, PLUS you have to deal with a dying tree that will fall apart on your building if you don't take it down in such a manner as to save the tree.
Another problem with oaks is they drop branches. Oaks seem to drop really big and HEAVY branches about once a year. I don't know why, but my experience is that it happens fairly regularly. Odds are double with two trees that you will get a branch through your roof.
My last concern about building close to any tree is what the roots will do to the foundation. If you plan on pouring concrete, will it stay in place 20 years from now with two trees right next to the building?
If this is the only place that works for you to build this, then think about taking out the trees. It hurts to do so, but a year from now, you will have trouble remembering that they were even there. If you have too much attachment to the trees, then take out at least one of them.
If one of those trees was removed, could you build wider? 16 feet is just enough to park in with no space for storage on either side. One thing that you will always want and need is more storage. There is nothing more frustrating then to spend thousands of dollars on a building then to realize it was too small before you even finish building it. An extra 4 feet wider won't cost you very much money, probably just a couple hundred dollars in lumber, but give you a tremendous amount of open space. Right now, at 16 ft, you will be crowded. 20 feet will give you some space. Of course, there is no such thing as too wide, but I think 20 feet is about the minimum for a workable shop to be functional. Mine is 24 feet wide and I really like that width. I also have a 12 foot lean too off the side for extra storage, and it's packed full of stuff.
Length is something that you can always add to later on by extending the length of the walls and adding more trusses. If you have the space, go for as deep as you can afford. If it was about not having enough money to do it as big as possible, I would seriously consider waiting and saving another year.
One year will give you time to plan it out to the smallest detail, prepair the area for building, and have enough cash to build it even bigger. If you have the money to build it bigger right now, then consider making it 30 plus feet deep. I know the tractor isn't that long, but when you add a bush hog to the back of your tractor, it's gonna take up allot more room. Taking off the implement just to park it in your shop will get real old, real fast. Figure 20 feet to fit the tractor with bush hog and have enough room to walk around it. Then you want to have storage room, or a workbench, so another ten feet works out pretty good. Mine is 30 feet deep and when I build my next shop, it will be deeper. At 30 feet, I can just do what I want in there, but I have to plan ahead of time what I'll be working on, and I can only do one project at a time. Right now I'm working on an old jeep. That's about all I can do in there, so anything else that comes up is either in the way of each other, or gets done outside.
Eddie
PS to Rob,
Awesome pics and comentary!!! I really enjoyed reading it and seeing how quickly you got it done. Thanks for posting them, I'm feeling motivated to go build something!!!!!!
Eddie
The tree thing is a tough call with no real "right" answers.