What size tarp would be good to carry in your BOB? In my other thread a lot of folks said a tarp would be better than making a tent out of contractor bags, so I'm trying to see what's out there as far as sizes and brands.

Also, how do you rig your tarp?

On the home page there is a good article about different ways to rig a tarp.
http://www.equipped.org/tarp-shelters.htm
Tarps are pretty versatile. The polytarps are cheap enough but they are pretty bulky.
Mountain Equipment Co-op sells good light ones in silicone treated nylon.
A bit expensive for throwing over boats, cars, or patching roofs and windows, but great for backpackers to rig as a tent substitute.
You can likely find them for less money if you shop around. Just be sure to buy one that is big enough to actually shelter you, your people and your gear.
I like this one:
http://www.mec.ca/Products/product_detail.jsp?PRODUCT%3C%3Eprd_id=845524442505763&FOLDER%3C%3Efolder_id=2534374302702975&bmUID=1231466409108

10x12 seems to work for me.
But to each his own.
If you go with a tarp you need plenty of good cord with it.

I carry an 8x10 in mine. I bought it at Walmart and it came in a plastic wrap / packaging. It fits next to my back. I like it and it has served many different purposes so far. My wife has one in her pack worth every bit of the 1 lbs that it takes to lug it around.

Nylon tarps seem the best bang for the buck - REI has a standard one, so does Campmor, 8x10 or 9x10 seems a good size to get pulled out and used alot (~ 1 lb). My current tarp is the REI brand, but that's because I got it really cheap at one of REI's .83 sales a while back. Kelty makes a really nice tarp, don't know the weight or sizes but I seem to recall they are blue and white and they have alot of grommets along the side for tying down (ck REI website). Silnylon is even better, but pricey. The nylon tarp isn't too heavy to hike in a day or two.

I carry a 6 ft. x 8 ft. Ultralight Backpacking Tarp in my get-home-bag. Packs small, big enough for one to get out of the rain.

I have an Outdoor Products Polyurethane Coated 12' x 9.5' Nylon Tarp that I got from REI during their annual New Years Sale. It was 30% off, and I used some of my Store Credit to get it for less than half price.

I tied some 550 Paracord to the grommets and use those for stakes and lines rather than the grommets.

Here's a similar Tarp Setup to what I put up except I use a centerline tied to a tree if at all possible. If the link doesn't work go to www.youtube.com and search for "Etowah 8x10 Tarp Setup".

I plan on adding a mosquito net to this setup.

My BOB has an Oware 6' x 8' flat tarp, 9oz, and 8 MSR needle stakes, 3oz, and 50' of line to tie out. Total 12oz. It covers me, my gear and my dog. Cramped but dry.

http://www.owareusa.com/
http://www.rei.com/product/734484

I do a 6-ish by 9-ish. I would love to find a good, heavy duty tarp in the 7ish square with a metalized side thought.

There's always one step up from the contractor's bag, a Christmas tree bag. I just got two more for a buck in the after-holiday sales.

They're 72x90"; opened, they've got a 45" diameter, 7.5 feet long. Poke a hole in the closed end to accept a cord, tie the cord between two supports, and you've got a big bag to break the wind and keep off the rain and snow.

No, they're not perfect, not very heavy-duty, but they only weigh about 8 oz, the enclosure bag is 9x12x~1/4", and will sit quietly in the bottom of your bag until you need it.

Sue

Another poorman's tarp are the plastic covers used to ship mattreses.If you have a local retailer with a warehouse they will have rolls of new bags on hand. A smile, $5 and you have thick mil, opaque plastic twice the size of a double/queen.kingsize bed.
A tarp is a excellent item to pack against the back for comfort.
You will save valuable time if your tiedown knots are pre tied ( there IS a tensioning knot, or use the plastic attachments)and at least a few groundstakes.
Fire and tarps- make them easy and quick. Once inside and warm your brain can nitpick 'shortcomings.'