Poor man's tarp can also be:
A piece of Tyvek. Comes in 9' by 100' rolls so the sky is the limit on size. It is tough enough for several uses and makes a good ground sheet. Use a good quality sealing tape to patch holes and wear points and you can get three or four camping trips out of one piece demoting it as it wears to less critical jobs. Last duty as as rock shield under the ground sheet.

A standard shower curtain works as a small tarp and, if sized right, as a useful rain cape.

Standard polyethylene sheeting, Visqueen is one brand, comes in black or clear and makes a good cover. The 4 mil stuff is okay for a couple of uses, a bit more if you patch any rough spots well with duct tape, but the 2 mil is pretty much one use only unless you get lucky. Black lasts longer in sunlight and doesn't act as a greenhouse. White, if you can find it, reflects a lot of heat and makes a decent sunshade on a sunny day.

Tightly woven nylons or Dacron, less subject to UV degradation, cloth can be coated with urethane to make your own tarps. Urethane coated and treated with silicone water repellent, Thompon's Water Seal is a favorite, it makes a tarp that sheds water well, lasts a long time and about half the cost of the commercial version.

I have both a 6x8' and a 10x12' silnylon tarp in my BOB. The 6x8 is for making a rain shelter while pausing from moving and the 10x12' is my shelter building tarp. it's big enough to build a shelter with a side or two if needed for weather protection of a sleeping/cooking area and big enough to be comfortable for several people if stuck in one spot for several days,
Back in my SAR days, I carried a 10x10 blue tarp on the out side of my pack. It was often used as a overnight personal shelter tarp for me and maybe a team mate (if needed) and as a team shelter if we had to hunker down or as a shelter while treating or packaging a patient.
I was picked on by team members until I used it as a 3 sided A-frame shelter one very cold and rainy 2 night training session while others used large trash bags. Those in the trash bag shelters were often wet and cold, while inside my tarp I was warm in my hand warmer heated 2 pound sleeping bag and dry. The 3 sided A-frame design also acted as a ground cloth and it was long enough for me to pull the ends in and basically close them off. During the day I opened the trap up and hung it on para-cord from trees as a team weather shelter.
Several others added 10X10' blue or the more expensive nylon tarps to their packs after that training.
As far as I am concerned, a large tarp in a requirement in a BOB bag.

Ditto on experiencing the tarp envy - they're great for setting up temporary relief from heat, cold, wet or wind. Kept an overheated Scout out of the sun for a couple hours, made a tarp believer there. More often its shelter from the abrupt downpour where you just don't wanta keep trudging along the muddy trail, stop, pitch a tarp, and watch the weather for a half hour instead. Best survival has us staying warm (not too warm) and dry, tarp helps with that.

In an emergency setting up a tarp has value beyond the shelter from sun and weather it provides. Set up a tarp and people think of it as a landmark. I have noticed that even if you have nothing more than a box of bandaids with a tarp it is an aid station. A tarp set up high enough to walk under, a table and makeshift seating pretty much puts you in charge of the scene. Try it some time. Don't be surprised when people start coming up and asking where you want them to set up.

People will naturally congregate around any such landmark and assume anyone who looks like they know what is going on is in charge. This can be a double edged sword. If you don't wish to be looked to as an authority don't put up a shelter or, at the very least, use a tarp in a subdued color and keep it low.

Six foot square, roughly the size of a standard rain poncho, is IMO about the minimum useful size if you want a lot of options. Rigged low you can sleep under it and have complete coverage if you sleep on the diagonal.

Eight foot square or over opens a lot of options and allows the simplest, swiftest and most foolproof of all shelters, the burrito roll. Simple as: spread tarp, lay on one edge, roll over making wrap tighter at foot than head. With practice you get complete coverage with all edges tucked under you even in hurricane winds, decent ventilation and end up with enough extra coverage at the head to pull in and shelter your pack. Total time: less than a minute.

Helps if you rig a light line from the foot inside so you can seal or loosen the foot end as need be. The burrito roll is a last resort but one that has saved my bacon when I was wet, exhausted, at the edge of hypothermia and there was no shelter anywhere nearby. Once your warm you can climb out and build a more substantial and comfortable shelter. Or not.

Eight by ten allows you to use it as a lean-to and ground cloth in one.

Twelve by twelve is about all one person can effectively use in my opinion and is pretty much the point of diminishing returns if your not setting up a base camp.

I'm thinking 8'x10' would be my best option. I have either my daughter or my girlfriend with me a lot of the time I'm out in the woods. I picked up a cheap 8'x10' tarp at Walmart for $4.88 today to throw in there until I find a better one that I like.

What are good options in the 8'x10' size that have ties on the outside of the ridgeline and a decent number of grommets on the edges? I'd like to have the ridge paracord outside the tarp rather than underneath it. Also, how many/what kind of stakes do most of you keep with your tarp?

Home Depot has an 8x10 blue tarp that has "blanket material" (their description not mine) on the reverse side. Haven't had to use it yet, but I don't think I would rely on it as my only blanket.

Have any of you have success using recycled Billboards or misprints from that industry.

The most common locally used material is a reinforced vinyl in heavy weight (last time I checked it was 6oz or higher). It is UV resistant, it's tear resistant. Of course you may have a big Budweiser, KFC or Corona print on one side, but I have yet to figure how they are disposing of those materials. I used to work with smaller signage and I have always thought of that material as tougher than the FEMA/Wal-Mart Blue Tarp.

Got a link? Sounds interesting.

Nope, sorry. It seems like a cheap almost brushed paper or tyvek material laminated to the back of a regular blue tarp. Check HD as Lowe's didn't have them.

Well, I seem to have gotten it exactly backwards. Lowe's now has them and HD doesn't. It seems they have gotten smaller though.

Here is the Link.