Another thing to consider is going with a loft style truss... http://www.rockyridgedesigns.com/truss_styles.htm
These are a little more money but you get an actual usable room. I am up in mine right now as I am typing this, it's 14' wide x 28' long and sits on top of my 26' x 28' attached garage. I built bookcases and storage into all of the knee walls so there is no wasted space. This is office space for my home business but I will do the same thing when I add onto the barn. It will make a great clean room for putting motors together and storing not so often used parts and such.