I made and use a Robinson Poncho Shelter a few years ago and use it when the hammock option is out. In fact, if you're hiking buddy carries one, you can pitch them together for a very roomy tent!

As far as size goes, I think mine is about 12' long, fully opened up. You want it to be able to get completely under the thing with a foot or so of margin at each end to stay out of rain/wind. You can pitch it as a lean-to, like my picture below, or simply cover up under it, like a bivy. I used mine last week while hunting in a day long drizzle. It covers my pack and everything.


I don't know much about the pre-made versions but this one works great!
http://royrobinson.homestead.com/PonchoPlans.html


Here's my setup: