I would look to @rotuts for solid information. He is much more methodical than I am (I lack a notebook and intention to start one!) and knows your smoker better than I do.
That said, if I am smoking a pork butt, I usually cut on in half to smoke 5-6 lbs. I smoke at 225-250f, and smoke until 195-200f. I give it 8 hours, but it sometimes takes less ans sometimes more.