The industry standard for the height of a shower head is 80 inches above the floor of the bath. This is 6 feet, 8 inches, which is suitable for most people of average height, and even usable for those who are taller. However, the particular shower head you choose will affect this height: a head with a long neck could potentially decrease the height by several inches.
Everyone Is Different
Because not everyone is the same height, the shower head height that works best for you (and the people who are living with you) will almost definitely deviate from this norm. If your household is taller, it may be best to make the height higher; if you are shorter, lower the height (or buy a head with a long neck).
It is probably best to start at the industry standard, and adjust from there until you reach a height that you feel suits your household best. If you are having the room redone professionally, your architect or builders may be able to provide tips and advice.