Pole barn design

I'd recommend 6x6's for the posts. You can notch them for the band beams making it easy to put together. As a former truss designer, if you buy trusses they can be designed to go 8' o.c. You'd only need a few but would have to run purlins for the metal roof. This would probably be the cheapest way to go.

With only a 12' span on the ceiling/roof and an unknown roof to match you could stick frame it. For stick framing you can use a 2x8 ceiling joist, 2x6 rafters, and a 2x8 or 2x10 ridge beam. No need to make any home built 'truss'. I'm not sure of the spacing between your posts but a 2x6 band would probably be undersize if they have roof load on them. 2-2x10s for the band would make the inspector happy.

Just re-read your post. If using a shingle roof you'd be limited to 24" oc for the truss spacing due to plywood deflection. Stick framed roofs are usually 16" oc but I'm not sure of the exact code regs for that.
 
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