Supertanker sized spacecraft
Orion-type craft have been proposed as large as 8 million tons (!), which qualifies it as "supertanker sized". This might be a little optimistic, but the potential for very large and relatively inexpensive vehicles is there. Can you imagine the Mars expedition you could do with 8 million tons?
http://www.angelfire.com/stars2/projectorion/orionsosed in the
