How much water should I add for pot roast in 5.5 crock pot?

You don't need much - the meat and veggies will release some liquid as they cook, which will flavor your whole dish. Browning the beef is the best thing - that really does add a lot of depth and flavor just by doing that! I only add about a cup or so of water - you can also add beer or broth - I have done all of those and they're great, too! Just be sure not to lift the lid off the crock pot very often - you want it all to cook together without releasing all the steam. And oh, yes, for some flavor add a smashed garlic clove, a bay leaf, a pinch of thyme or rosemary, and some salt and pepper! Enjoy!