Savannah monitors do not get 6' or 200 pounds. Three to four feet is average. Large one's get to five feet, and are quite rare. Big chance that you did not see a savannah monitor.

Walking, no. They can escape quite easily and are quite fast. You could lose it then your lose monitor would be considered a "nuissance". And if caught could possibly be put down.

Make sure they are legal in your area/state as well.