24" o,c, 2x4 for load bearing wall and rafters for workshed?
Although the modern trend is fast stuffing more &amp; more lumber in the wall, walls framed with 2x4s on 24" centers will support nearly any sort of roof load you're likely to ever see - especially if you don't live in a far northern climate. I've seen 2nd-floor hot tubs with far less support than that.

You do have to be pretty careful laying out the studs; 1-1/2" isn't a huge surface to stitch two piece of OSB together on, so if one end of a stud is kicked to the side only 1/2", all you have left for one of those OSB sheets is a scant 1/4" of nailing surface.

For roof sheathing, you might want to go a little heavier, though. I've built two matching roofs using only 1/2" sheathing (this was in Albuquerque), and it was really pretty uncomfortable to walk on such a flimsy roof. Come to think of it, the trusses in those roofs were 24" oc, which was fine for that climate. It was really hard to drive roofing nails into, because the sheathing flexed so much.