Doesn't matter too much. I'd go for big chunks so they don't disappear entirely when you cook them.
Cut the ends off the onions, peel them, slice them in half across the middle, then cut each half in quarters, so you end up with 4 large wedges from each half. They'll be very limp by the time the pot roast is cooked. I'd use 2 or 3 yellow onions, preferably sweet Texas Vidalia.
Peel the carrots. Buy fairly large ones. Cut in pieces about 2 inches long, and cut the really thick chunks in half. I'd use a lot of carrots, 4 or 5 big ones.
Water will evaporate. Don't worry until the juices are getting very thick, then add a cup or so of water and stir the sauce. The object is to prevent the sauce from getting burned, which can happen fairly quickly. BUT, you don't want it soupy -- you want a medium thick gravy. So let the liquid level drop, and add water when it's down to an inch or two -- at the end, let it evaporate until it's as thick as you want it. If it's too thin, remove the meat and veggies, keep them warm, and boil it down a bit on the stove top. It's easier to do that than it is to un-burn the gravy. If it's REALLY too thin, you can always add a bit of flour or cornstarch.
Now just go do it! You can't screw it up! Trust yourself, and use a little common sense.
