The silk sleeping bag liner I am using now weighs 4 oz.
It is 78" long and about 30" wide at the top. I will try to calculate the area, but it is a mummy shaped liner so I'm not sure how accuate my calculations will be.
As to breatability, I am trying to minimize condensation. I expect something like cuben or space blanket (mylar?) would likely be lighter but it is my understanding that those materials do not allow water vapor to pass thru.
The sleeping bag liner works very well, but is too small in the shoulder area as I found out this weekend when it dropped to 29. I need a little more coverage.