>>582178A square tarp is very versatile. Look for one with lots of solid attachment points. Mine has 19 and it's a joy, I can even set it up as a closed ground tent. As for the size, 3mx3m is plenty enough. Smaller is a bit short. Pic related, mine this summer. Dun goofed and pitched an A-frame because I thought the storm would come later the following day, naturally it came at five in the morning. I still stayed dry. Had I gotten a smaller tarp, the A-frame would have been slightly too short and rain would have entered with a bit of wind. A diamond pitch would still have been enough though.
I'm also
>>581420 by the way.
