Re: HFSS AIR BOX PROBLEM
If your antenna is planar, you should be using an unshielded MoM tool, no worry about where the box is, and you don't need to mesh the volume, MoM tools just mesh the surface of the metal. I would recommend Agilent Momentum, it is not the fastest tool, but from results I have seen, it is the most accurate unshielded tool. (I work for Sonnet, a shielded MoM tool vendor and occasional competitor with Momentum.)
If your antenna is not planar, then you must consider the comments of the others above carefully, but I would add one more thing. When you have the box big enough, its exact size will no longer be important. If you have to be sure you have a good result, then do an analysis with what you think is the best box size. Then do a second analysis with another box size. If the change in the answer is equal to or larger than the variation you can tolerate in your design, then you must make the box even bigger. This is the approach we recommend when designing planar antennas using our shielded MoM tool, works quite well. Should work for volumen meshers too. Good luck.