Not the answer to the question you asked but IEC 60601-1 provides a bit of guidance. They say that it is "difficult" to define the dimensions but indicates that 1.5 meters away from any point in the environment has been "justified in practice." (So if a patient is laying on a table, the patient environment includes the complete table; i.e., so 1.5 meters away from any point on the table). I can see a number of difficulties trying to exactly specify.