Start with a rump or chuck roast. Here is what Cattlemen's Beef Board and National Cattlemen's Beef Association says about pot roasts:
For everyday family meals, casual gatherings, and for the health-conscious, the round and bottom sirloin cuts are leaner and economical. Moderately priced roasts include tri-tip, round tip, rump, bottom round and eye round."
So I used a bottom/rump roast because that is what I found. But you could also use a chuck roast or a round roast.
I recommend a 3-4 pound roast for 4 generous adult servings or 6 average adult servings.
Next you want to trim the fat off of your roast. See that thick white layer on top? That is fat and its usually easy to cut off.
Once the fat is too close to the meat you can leave it like I did below. That is very little fat and will give it some flavor too.