For length, the manufacturer's recommended size is largely dependent on your weight - since you use your weight to flex the ski, weight determines your turning power. Using your height and ability as secondary factors, you may want to add or subtract a size. Particularly, some manufacturers state the weight range for each size, and you may be within the recommended range for more than one size - in that case, if you are short (less height = less leverage), a beginner, or you want a ski that does short turns, you might want to try out the smaller of the two. But your best bet is to ski various lengths and decide what works best for you.
By the way, yesterday I went up to Killington for the telemark demo being hosted by Mountain Travelers
. I demoed a pair of 176cm Ethics, mounted for AT with Fritschi Freerides, and they felt just right for me. I'm 6'3", but only about 147, and an advanced skiier definitely not an expert. Ethics might be OK for you - particularly if they are mounted AT rather than Tele. One of the comments about the Nunyo in Couloir was that "bigger boots and bigger skiiers can overpower this ski when pushed" - that comment probably applies double when mounted AT
Mountain Travelers should be doing another demo in January and one in March. I don't know if they're going to have AT skis for demo again; they are a telemark/AT shop, but there is not a lot of demand for AT on the East Coast so they don't have a full demo fleet. For AT, they did have one pair of Ethics and one pair of Crossbows yesterday - both were 176cm. When I spoke to their guy on the phone a few weeks ago, he made sure to emphasize that they MIGHT have ONE pair to demo.