First of all it doesn't matter how big the roast you buy just as long it fits in your crock pot. With your roast make slits on the roast, with the fresh garlic chop it in quarters and add one by one in the slits.
Add your roast in your crock pot and add all your vegetables in it and after that Add, Water until its covered your vegetables and roast. Add all the ingredients into the crock pot.
If you like start early the day you want to have it. That would be fine, the early the better.