I have a wire (covered in a coating) four story habitat for my two rats. I suspend it from the top of the habitat near the top level. Neither of the boys like to hang out in it. The rat that I had before them LOVED hammocks.
I probably wouldn't go much larger than an average domestic 5-6 month kitten. These hammocks are pretty durable, but 12"x12" can only go so far. I think it really depends on what you are suspending it from.