rob_labI'm not sure about the dimensions but I'd say it's not really a whole lot larger than a 2-liter bottle of soda. As for your other question, it comes with a plain steel cap with a rubber gasket. So, you can use that to fill up your keg then replace it with the tap later.
rob_labIt's 5.25" in diameter and when stood up is 18.5" tall. The handle is removable and can accommodate shorter or taller custom handles. It can also be stored on it's side for storage in fridges that don't have the height.
Can the tap be removed for transport with the top still sealed?