It really depends on what you like and what you are doing. If you're doing bc then yea above your head would be fine. if its the park then you could follow the whole chin eye rule or whatever it is. For me, i love longer skis, ie: 185cm for the park. It really comes down to preference though.
if your a kid an still growin den go sumwhere within 3cm each way of your actaul height, stopped growin= about da same for powder, cud maybe go bit bigger, normal cruizin= 10cm shorter. and if u ski anything about 5 to 10cm shorter, park is up to what you like, well i guess all of those are but more so for the park skis, id go about ur own height for park though