[wdfifteen]

By this time tomorrow I will have a 40x60 barn. It is not quite a pole barn, it has interior supports ten feet from each of the 60 ft walls with beams paralleling the walls. I want to wall in half of the building - a 40x30 space. It will be 30 feet deep, and the 40 foot dimension is a 20 foot open area with a ten foot space on each side that has a 10 foot ceiling now. I want to install a ceiling over the 20x30 foot area, so I will have to have something that spans the 20 foot open area to attach the ceiling to. I'm thinking trusses, but how do I size them? All they will be supporting is 600 square feet of drywall and some lights.
Suggestions?