So it looks like you put the center poles a little off-center to make more room in your aisleway. How far apart are your poles and center supports? And, though I don't think it'll be an issue with the size we're wanting, but what is the farthest you should put poles and center supports before adding trusses or at least another support?
Here is the very simple plan that I used as a base design. If you look at this plan, you can actually go to 28' in width without center posts at all with the sets of posts every 10' along the length. Since we wanted to partition ours with a shelter, and I wanted to avoid having 2 posts in the shelter area, based on another plan I saw, we went with 28' in width, but increased the distance of the posts to every 15' in length (total length 30') so the shelter part is 10'x30' and we added the 3 'offset' posts at the 10' width of the shelter. Since 16' is a common lumber length, this gave us 30' in the barn/shelter with a 1 foot overhang on each side. Similarly, even though it's nice to have long overhangs, since 16' is a common length, we used 16' 2"x12" rafters and just reduced the overhang. Just more convenient, lumber wise. The only really important factor in the spacing of the posts/purlins and roof pitch is to consider the snow load that you need to support. Although we basically stuck to this plan to be conservative, it is really overbuilt for the amount of snow we get in NC, e.g. We could have used 2x10 rafters instead of 2x12, less pitch in the roof, etc. It was impressively sturdy, however, during the wind and rain we get during hurricane season.
For the exterior walls we set 4x4 posts evenly spaced (every 5' or 6') to support the girts/siding (see picture). It would be similar for your interior walls/stalls.
BTW, I said my wife and I built this by ourselves, but I forgot to mention that "Cinnamon" did offer to help store the grain (last picture)
Finally, although I had built decks, small sheds, and remodeled our kitchen/baths, I had never taken on building a barn, but I'm sure glad we did. It was well worth the effort and seriously within the capabilities of DIY.