6 sq meters of groundshete will start to be heavy and bulky. You don't say what your constraints are in that respect. Also, it is best for the groundshete to be smaller by some margin that the actual coverage of your tarp if that is what you are using. You don't want rain to drip from the tarp or be driven by wind on the edge of the groundsheet and to form a pool with you in the middle.
For smaller size I like the space blanket (the thick one, found at Bensbackwood for instance). A full one is already a large coverage. I usually take a half one (cut lenghtwise).