Space 24" on center and use 5/8" apa rated sheathing. Max span depends on pitch, member size, and loading. My alpine truss span table shows a 2x4 common truss with a 3/12 pitch can span 40' supporting a total load of 40 psf.
Above advice on the span it right. See your truss guy/gal. Trusses are usually 24" OC, but they don't have to be. 2x6 studs are likely OK, but it depends on your snow/wind loads and ceiling height. My trusses will be 2x4 and 2x6, 24" OC, span 42', with a 5:12 slope. I would never use less than 5/8" ply on the roof, and clips, as mentioned. I am not a fan of OSB anywhere; it you get a roof leak, it may well turn to "oatmeal" (quoted from Martin Holaday on greenbuildingadvisor.com). I would have said "sh**" but that is too rude for some forums. Make sure your foundation is skookum, too; that span could generate a significant load.
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