Flat tarp size

If you're looking to use a tarp to go very light when you can depend on the weather to be tame, then I'd suggest going as small as you need to cover yourself. You can always pitch it very high and it will still be roomy.

However, 8×10 is the minimum I'd want to depend on shielding me from any inclement weather. Pitched with edges to the ground it'll still feel roomy for one, but you won't use all of the space efficiently the lower you pitch the peaks. The lower the side panels get, the less wiggle room you have under them. An 8×10 is comfortable for two as well, but again, this all depends on the pitch.

A square tarp, as opposed to a rectangular one, will give you the most versatile pitching options since it's symmetrical. I haven't used one so I don't know what the ideal dimensions would be. I'm 6'3" so I like to have the 10' length of my tarp for splashes, but a 10×10 would bee too large/heavy for me. Something between an 8×8 or 9×9 could work for you. Some square tarp pitches provide more enclosed shelter for less length. Keep in mind that for the most versatility in pitching, you may also be carrying a bunch of extra guyline.

If the real appeal is the ability to set up different configurations, you'll likely get more out of a square tarp. I went 8.5×8.5 because that's the current limit of cuben options. I have the the HMG tarp. With sil you can go larger, but the weight can become significant. I decided that at the point where a large tarp started to weight the same as a mid, I'd rather have the mid for bad weather.

10 feet long minimum, trust me you want 10 feet when the wind is blowing the rain.

Because of my numerous negative experiences with military poncho / tarps for well over a decade, I have never used a tarp in the civilian world. My ARMY poncho / tarp was 54 1/2" x 60". Not nearly big enough to protect most of my 5' 8" body from any blowing rain in any configuration. Also so short that from my knees down would have to stick out in order for my face to be protected from slightly blowing rain.

I found that even when pitched fairly close to the ground, slight winds blew rain in a good 1 1/2' under the ends. so I'd personally suggest a length that's a good 5 – 6 feet longer than your body when used in fairly open terrain, a little less when in very sheltered terrain. The width would depend on how many people will be covered with it, I'd suggest no less than 55" per person for calm back sleepers if leaving gear outside and a minimum of 2 feet more with gear inside. Though as most of us carry very little extra gear, the pack can be slept on in most cases.

For me personally, that would mean a square tarp that's 11' 8" on a side which would weigh about as much with a ground cloth as my Tarptent Contrail, so not an option.

I agree that a square shape will allow many more pitching options as the ARMY tarps that we used could be snapped together for 2 people, and gave both people better coverage.

A lot of your size requirements will depend on gear inside or out, back or side sleeper with bent legs, and the sheltered or not environment it will be used in.

Hope this helps.

Dave

For a square tarp, 9'x9' would be the minimum for me with 10'x10' being about right. I use a 9'x7' currently and I wish it was longer when the weather gets wild.

Ryan

Read this fine article
http://www.backpackinglight.com/cgi-bin/backpackinglight/tarp_camping_inclement_conditions.html

I use an 8×10 which does give you extra room on the ends

I had a custom 10.5' x 7'
Am so far am very happy with it.

I think a 7×9 or 8.5×8.5 is the perfect solo size. After getting used to a 7×9 i couldn't imagine needing a bigger size.