if youre just starting, go 171, but i am 6' too and ride 181 ar6s. but i like longer skis. if you progress fast, i would go 181s as well, it helps with ballance all around the mountain..
i duno all depends what you prefer and what your doing. if you like the tech stuff on the rails and fast spins then hit up the 171 but if your guna be hitting up some more all mountain than your going to want to go with the 181