Hi I'm 180cm tall, weigh 78 kilograms, and ski on the Dynafit Stoke 173cm with a 100mm waist. I like them cause  they turn easily and the skis rebounds nicely into the next turn. 

When the powder gets really deep it would be nicer to have a longer and wider ski for more float.

I've been out in the backcountry 22 times this winter and they performed well.

The 186s might be harder to turn in tight trees.

Ski length is not what it used to be.  At 173 cm height and 73 kg I have skis form 177 to 186 cm long.   Many factors come into play when deciding what length ski to buy.  The amount of tip and tail rocker , the camber or reverse camber, turning radius, and the stiffness of the ski all play into how long a ski will feel.  My shorter skis are stiffer and have less rocker than my 186 cm skis. Your height will be less important than your skiing ability, style, aggressiveness,  and weight.

So in short, it is impossible to say with the information you've provided!  Read more here:

http://blistergearreview.com/r.....ski-length

Talk to your buds, search the web for opinions, see what length others around your size/style/aggressiveness are riding.