I agree with the others about the pitch. Looks to me like its in that range. The real question is how much snow do you get and what is the best pitch for YOUR area.
Don't build something because you like the way it looks, but build the best building possible then make it look nice.
The cupola is realy simple to add to the framing once you get to that point. If you want it to stand out, I'd go with four feet sides. If it comes out too large for your taste, then scale it down to what you think looks the best.
You might want to do a search on sliding doors on this site to read what people who have them have to say about them. Very few people like them, and if your building new, there's no good reason to put them in.
I put a roll up door in my workshop. Not very dificult to do, looks great, very functionals and reasonably priced. Garage style slide up doors are another good choice. I considered them both before buying the roll up.
As for large swinging doors, I'd be hesitant on them due to their tendancy to sag over time. It's very hard to build a large door that wont be affected by the weather and it's own weight. You frame will need to be reinfornced, with some very large hinges. If you cut a single corner in their design, it will be your failure point.
For me, I don't like going back and redoing anything, and that's my fear with the large doors. Just because one person might reckomend them, doesn't mean they will work for you or that they will last more than a few years before sagging.
Another good idea is to seach this site for barn plans along with google or any other search engine on the net. There are some free plans out there along with plenty that you can buy.
Good Luck,
Eddie