I just pulled the trigger on a 29er frame. I'm going to build it up with a tour on the Great Divide Mountain Bike Route as a goal. I want to put disc brakes on it. I've never had discs and have very limited knowledge. People recommend mechanicals for this ride. They say they're easier to deal with when you're out in the middle of nowhere, which is where I'll be on this road. I'm considering Avid BB7's.

In poking around I've discovered that these come with different sized rotors. Great! Now what?

Does anyone have any advice? I weigh 200 lbs. I would be carrying a touring load - camping gear, clothes, food for up to 3 days, etc. The route is mostly forest service roads, but with some singletrack. There are plenty of passes to climb, and then ride down the other side.