Sequoia, it would be simplest to just cut your roast in half, and freeze half, putting the other into the crockpot. I'm thinking using only one pound of roast (shoulder chuck is good, for example). Since the heat is only on the base of the pot, your root veggies, cut up, go in the pot first. Add a cup of water or beef broth (lower sodium is safest), and the beef on top, with slices of onion on the beef. Cook all day on low. Fork tender is the goal, but leave the lid on for several hours before checking doneness.
Liz, a true slow cooker has heat up the sides part way. They're usually shaped differently--lower and broader, and can handle a 7 pound roast. You want a slow cooker to have one third of it not filled. I believe a crock pot can be a good 3/4 full. Sigh, I have had them bubble over...wasting a lot of time, dipping out food, etc. This is only a general guide--I know slow cookers do vary in size, and I have a distant memory that the same is true of a crock pot. Mine are "average size".