In general, the larger the tumor the greater the chance that cancer may be left behind if the entire kidney is not removed.  There is no exact size cut off, but smaller tumors are most often removed by partial kidney removal.  In addition, tumors that are located along the outer edges of the kidney are also generally more amendable to partial kidney removal.  The reason for this is that the blood supply to the kidney enters at its center. 

If a tumor is near the center then there is a higher chance that the blood vessels may be damaged if an attempt is made to remove the tumor only.  On the other hand, if the tumor is located on the edge of the kidney then there is a much smaller risk of injuring the blood vessels that supply the kidney.