I think that sleeping crossways seems like a good idea until the detail is looked at.
Say you want only a standard double length of 75", then add 1" of mattress clearance (to get your fingers down the side without amputation) and 2" of sidewall thickness each side and the overall body width needs to be at least 80" - but I wonder how many people think instinctively they can sleep crossways in a 72" wide body?
The 80" length of a queen or king size needs a 85" wide body, which is getting pretty substantial.