Choosing A Ski Length

Question here: I'm 5'9 180lbs. I prefer powder and off piste in and out of trees. I bought the Line Sir Francis Bacons 2012 in a 184 length but haven't mounted them yet because I'm thinking of switching to the 178's.
The 184's look really long and I'm looking for something that will be quick in the powder and trees. Any suggestions?
Also how should I mount these? I don't ride switch but I would love to learn.

I hope you got your answer by now...if not, let me give you my $0.02...
I'm, 6'4" and 210 lbs. I ski 184 Atomics...usually Savage Ti, but also have the original Crimsons I like to get back on. Neither is a great tree ski. If I want trees, I have some old Metron M9s in 178. You are much shorter and lighter than I am. You should be on a 178 or less to ski trees, but still need to consider the turn radius.

When you're talking height, are you talking in bare feet or with ski boots on?

Please consider the idea that your skis don't know how tall you are; but they know how much you weigh and how well you control and work the entire length of the ski. Also "ROCKER" has significant influence when it comes to "pivot" "swingweight" and the skis ability to master crud, powder and less than perfect snow. Demoing does not always yield ideal results. Buy your skis from a reputable shop that will honor a performance guarantee and take the skis back if you hate them (allowing you to choose a different ski, not get your money back, lets be serious) BE HONEST ABOUT YOUR ABILITY, and where you actually ski. Make sure you buy a boot that you can flex and achieve a good range of motion so you can control your skis without endangering your knees so you can ski all day and really enjoy your new boards. Even the best ski racers have coaches and people watching/helping them achieve excellence, so don't be too proud to take a private "session" with a professional instructor once in a while. You might actually become the expert that so many skiers THINK they are. :-}

How do you choose a junior ski size?

For KIDS . . Beginners, never above shoulder height. The tips cross easily if the skis are longer, impacting the fun and confidence levels of the child. Once the kids are out of the wedge "no more pizza" you can go to the chin and gradually size up to "nose high." Safety and fun are paramount so a little shorter is always the safer way to go. Some parents think their kids are experts simply because they ski expert runs. (Sleeping in the library does NOT mean you are a book) Longer skis put more strain (torque) on joints and bones causing needless injury. Finally; Junior racers have the benefit of the manufacturer's suggested lengths on their Race Brochures,

What a Joke...

If you already have a ski that you feel is the right length for you, consider where in range of sizes for that model your ski fits. When you look at other skis start with lengths in the same place in the range. In other words if you're favorite ski is the longest offered for that model, you'll likely be happy with the longest ski of other models.
While weight and height do matter, ability matters a lot more. And weight has more to do with how stiff a ski you need than with how long.