It either means 6' of cargo space, or a 6' bed in that sleeper. On '06 and earlier Sprinter, there is 165.9" between the back of the seats and the inside read doors. If it's a 6' bed, 72 inches, plus half an inch for the bulkhead, that leaves just 93.4" for cargo. That's enough for a couple of 40x48 skids, but that's it.
48x48 skids are out of the question. That would really limit what that van could haul. Not many carriers will accept a van that doesn't have at least 96 inches of space back there, with most wanting 108".
Slow and steady, even in expediting, wins the race - Aesop