It all depends on what you are trying to tile. For some applications 4x4 tile will be big enough or small enough for others it might be too big or two small. It mostly depends on what you like the best for what you are trying to tile. Its mostly a matter of preference
Depends on the purpose. If you are doing countertops, 4"x4" is the perfect size. However, if you are tiling a floor, you would want to use 8"x8" tiles or even larger 1'x1' tiles. As I said, it depends on the application.
