There is definitely 2 variables of length and stiffness to consider, and I would correlate it loosely to length should match skier's height and stiffness matches skier's weight.
But both variables are also linked both to each other as well as to skill. Where reducing length or stiffness==less skill, and increasing==more skill.
AND reducing one variable and raising the other can cancel each other out to some degree.
Getting a longer ski can also compensate for
a) more weight
b) more height
But having everything match in both is better then compensating.
I think you can say is somewhat like a car's suspension. If your springs/shocks are really stiff, you can have soft tires to give you a comfortable ride. And the reverse is true too where harsh tires can be compensated for with softer springs. But dialing in everything to match both the road you are on and how you drive will result in a better performance envelope.
In the past there weren't quite as many skis to choose from, so you could only make adjust mainly using length; but nowadays there's a plethora of skis that accounts for many styles and stiffness and widths for the same length ski.
So here's my take:
In general, I'd go by the skill level should be matching the length of the skill as they do in the rental shop from the chin for beginners, up to your height for experts.
Then the stiffness or ski-level can be the secondary pass that then does adjustments from there for your weight.
Let's remove the variable of learning and skill. Say you are an expert and are done learning and are dialed in, and you know you like to ski skis about your same height. Let's just evaluate some skis.
Here's an example.
If you look up all the user reviews of the bonafides from esteemed reviewers here. You see that the "lighter weight" 150lb people are saying this ski is too stiff for them, they prefer the kabookie. However, the reviewers are inevitably 6' tall and are skiing bonafides in 180 or 187 length but still weigh just 150lbs, which I feel is the main mistake in the ski selection.
If skill only equals length as people have said; and experts only means long skis; then who the heck is supposed to buy the 166 or 173 versions of the bonafide? Why do they even make the bonafide in these sizes? The 6' tall expert skiers surely are not going to ski a 166cm ski no matter how skinny they are or how "expert" the ski is proposed to be.
Instead I think you need to match length approximately to height, then get the proper stiffness to somewhat match to weight.
Assuming skiers are both technically proficient and fit and ski in the same way (style).
Skier A: you are 180cm tall and 150lbs. You have a BMI of 20, which is technically "normal" but I feel is somewhat bean-pole-ish skinny for an active person. Perhaps the Kabookie in 180 works right for you, as your mass is relatively light for all other people of your height.
Skier B: you are 166cm tall and 150lbs. You have a BMI of 25. Assuming you are actually fit, and that's not all flab, you have more muscle and mass for your given height. You need a proportionally beefier ski then skier A, if you get the ski that proportionally matches your height. So perhaps the Bonafide in 166 IS the right ski for you.
Skier C: you are 166cm tall and weigh 120lbs. You have a BMI of 20. You match skier A in proportion and perhaps the Kabookie 166 is right for you.
So that's my take, I think basically consider matching length to your assumed skill level based on height. Then assuming you are fit, use your BMI to calculate whether you need something beefier then "normal" or lighter than "normal" (although I think the BMI's measure of "normal" needs a moved over a couple of points for people who are athletic).
Edited by raytseng - 5/24/14 at 9:43pm