It’s wingspan is said to surpass that of the Hughes H-4 Hercules “Spruce Goose”.
Which, once it achieves it’s first flight, would give it the largest wingspan of any aircraft in history

Microsoft co-founder Paul Allen rolls out Stratolaunch plane for launching satellites. Plane flies up & launches small rocket w sat to LEO pic.twitter.com/9e8hp5EmP7

— Breaking Defense (@BreakingDefense) May 31, 2017