By your question, I'm assuming you need to make a rectangle that is within the boundaries of the tile. I just installed tile in my home with this problem. What I did was buy a 7" masonary cutting blade for my circular saw. After marking the area to be cut I clamped the tile so the cut would fall between two 2x4's. I then made a plunge cut through the tile on the long side of the rectangle and finished the cut to the corners. I repeated this for the other long side. I then used my tile scorer and scored the short sides just between the saw cuts. Take a towel and fold it making sure its flat. Place the tile on it and carefully tap the center of the cut area with a hammer. I was lucky to get it to crack in the middle and snap on my scored lines. This may work for you as well. Good luck!