8 x 10 blue tarp is SOP for any of our kits. They're so inexpensive and versital it's almost a sin to not include one.
With a blue tarp, duct tape, and zip ties you can fix just about anything in Alaska.....lol

I don't have this but out Scoutmaster does, and I consider it the Cadillac of non-silnylon tarps - 9x9 from Kelty http://www.rei.com/product/627834. If you look at the picture you see plenty of tie outs, they are all strong and reinforced, and the tarp itself is leakproof in the heaviest downpour. Three adults and a handful of Scouts passed a cold, wet evening comfortably underneath the larger size. $60 is not cheap, but it provides a lifetime of use. The REI brand tarp I carry is the same size with reinforced grommets, cheaper but no longer available apparently.

Most places we hike we don't need stakes to tie down our tarps, plenty of trees and root material. If you had to, use your hiking poles and a couple lightweight stakes.

i did not read every reply to your question but i would take the
largest tarp i could stuff into my BOB..i took an 8x10 tarp on a canoe trip many-many years ago with the idea that would sleep under the canoe with the tarp to compleat the shelter.i ended up using trash bags opened up to give myself more cover.

For you folks with silnylon and similar thinner materials...how well do they hold up to wear-and-tear? ...especially those of you using them as groundcloths?

i have a large Baker tent made from Silnylon and it's held up in very bad weather.it's sort of slippery so i never thought od using it for a ground cloth.i use a space blanket.

here is that shelter after a heavy storm.i woke to find a very bad thunderstorm sweeping across my camp on a open point on a lake.the shelter had pulled the stakes and was flapping and pulling on the main tie-outs.i cut the lines,pulled it down to cover the gear that was inside and put rocks and branches to hold it down and got back in my tent to get out of the driving rain.i took this photo later when things cleared and i was getting ready to set it back up..nothing ripped---

Check out tent "footprint" tarps. They are no too expensive and come with grommets, ties downs, etc.

I have one wrapped around an REI minimalist bivy with an REI Travel Sac inside, all covered by a Sea-to-Summit waterproof compression bag.

if anything I would like to trade up from my 7 x 7 tarp to at least 9 x 9.

For those of you that use a cheap blue tarp you might like what Tinny turns one into. This first video shows it done and the others are how to make it.

http://www.youtube.com/watch?v=0fXDcFfNwLs

http://www.youtube.com/watch?v=NnO4Nplk9m8
http://www.youtube.com/watch?v=3vRceEXBm3M
http://www.youtube.com/watch?v=hUHRFnJ7JJI
http://www.youtube.com/watch?v=lmWtZZvD22k

hope you enjoy!

But if you are looking for a lightweight tarp and are handy with a sewing machine there are many lightweight backpackers that make their own out of silnylon and some go as far as making a hammock to go with it. It is amazing what you can make very easily and cheap.