I just got a Driducks poncho. Here's the dimensions:
After I cut off a sort of non-detachable stuffsack from the poncho, the weight is 7.3 ounces, which includes all the standard poncho hardware. The main dimensions are approximately 87"x52". There is a 3/4"-1" hem around all the edges, and 6 lightweight plastic snaps down each of the long sides (3 male/3 female per side). The head hole is roughly D-shaped (if I were to cut the hood off, there would be a D shaped hole in the middle of the fabric), and measures (when laid flat) approximately 10" wide (along the spine of the "D") by 6" deep.
The main body of the poncho is not a single piece of fabric, but the front and back are separate pieces of fabric with the seam across the shoulders. However, this is a heat-sealed seam, and I don't doubt its strength just lookign at it. I found one flaw, which is that on the back side of the hood, there is a 2" section where one side of the fabric didn't get completely incorporated into the seam. I've seen this sort of flaw before on Driducks raingear, particularly in curved seams, like the crotch of the Driducks pants I also own, so I don't think I got too terrible of a lemon.
Currently I'm thinking that there's plenty of fabric in the hood to just pull forward and use to sew the head hole shut. 87" seems plenty long enough for a bivy, though I plan to use a much more breathable fabric like 1.1 nylon over the head area. The main concern I have is how to best mate this material with the silnylon I intend to use for the bottom. From the little experience I have, Driducks laminate loses strength when there's a line of needle holes in it.
Also, I don't normally do this, but the place I bought the poncho had a good enough deal that I'll mention them here. http://www.thegearlessmachine.com/driducksponcho.html $13.99 and free shipping.