bunk is roughly (+/- .5 inch) 24X79 in size (2 feetX6.5 feet) so it would be a snug fit and it weights roughly 200 pounds.
from what I read on their site, it is installed by them at their site. Not sure if you can buy one un-mounted. It does look small but who cares if it meets DOT requirements, isen't it going to be mounted on a truck to light to matter? Of course, why spend the money on a sleeper if you can not fit comfortably in it.
I've seen some sprinters with boxes mounted on them, funniest looking trucks I have seen, they are bit narrow for the boxes I have seen mounted on them.
The main reason they are importing chassis' is for the emergency services. They had made several amulances out of the regular sprinters and have found them to be to narrow (same reason they stopped using regular E350 vans).
I don't know that I would ever own a sprinter, I am rather partial to the powerstroke with the right tweeks you can get 22-24 MPG out of them as well, and with 8 bolt wheels and a floating rear axle they tend to carry the weight better.