If, during this survival hunt, you intend on packing everything in over long distances on your back, go with the smaller (lighter) 3m x 3m. If, however, you aren't going to be going long distances with everything on your back, then go with the larger (more coverage) 4m x 3m.
The reasoning being, the smaller one will do just fine for precipitation but may block less wind. The larger one will give you more space under the tarp and offer better wind blockage, but will be more of a pain to pack long distances.
Anyway, my two cents...