5'6"-5'7" is about 167-170cm. For park skiing the general rule of thumb is to get a ski that is equal to your height. Of course you a go a little shorter of longer based on your preference, but it is the most common length and best place to start. Again, this is for park skis. For more all mountain and pow skiing, the average is usually slightly longer than your height; say +5-10cm. But that varies a lot more than park ski length.