The halacha is to place the mezuzah at the begining of the top third of the doorway. As long as it is at least one tefach (handbreadth, about four and one half inches) from the top of the doorway it is kosher. If the doorway is very high, the mezuzah is placed at shoulder height. What is considered very high? If the mezuzah would be above the face of a normal person, the doorway is considered ‘very tall’ and the mezuzah should be placed at shoulder height. The reason for this is that mezuzos are meant to be seen, so that we remember Hashem and his torah.