I have droppings boards too (e.t.a - that is, I have them instead of a droppings pit), and love it. I prefer to scrape the poo off into a bucket every morning. I use an old dustpan to scrape with, but other people often use a hoe. That gets a LOT of the chickens' output removed from the coop...
I don't think I understand the question about how wide -- you make it about a foot (on each side) wider than your roosts, no? Like, if you have a single 6' long roost, you would make your droppings pit approx 2x6', centered under the roost (or a bit wider, to simplify things by just running it...
My chickens can poo about 8" to occasionally like 10" from the roost when they're up there, so I would say size your droppings pit to that sort of range.
Pat