I built a Viking-based frame for two riders each 190cm (6'3") tall and it came out to around 3.6m long - that's 11 feet 10 inches in the old money. You can see more here:
http://forum.atomiczombie.com/showthread.php/7973/page3
Mine is a quad with a Loderunner rear end rather than a tadpole trike like the original Viking. You can knock off maybe 20cm or 8 inches for the equivalent Viking length. But it's still a monster...
Regards, Neville