Wow, that's funny you started this topic. I was just about to start it myself.
I have the same question and through some testing, think I know the answer.
I'm a 5'7" (shorter legs/longer torso) 145lb male skier. Probably level 7.5-8ish. Three years on skis. 30 years old. Athletic.
I just purchased some Dalbello Krypton Pro ID boots. I had done a ton of research and decided on this boot as I wanted something that can take me through the next 4 years and be super high performance for all mountain/off piste skiing. I really dig and believe in the technology behind the 3-piece design. In saying this, I knew a boot at 100 flex (setup at easiest flex) would be a challenge for me. They have the Gold ID liners which I read (on this site) add ~15% more stiffness.
At room temps I can get the boots to flex OK, but not as soft as I'd like them though--and I'm not even in cold weather. Soon as I hit the cold weather this season I'm having a very hard time even flexing the boot forward. This is even true when standing still on the skis and trying my best to slam my shin into the boot. By doing that test, I know that it's not my skiing 'ability' that's lacking the ability to flex the boot. I simply can't gain enough leverage with my sawed off short legs and 145lb weight. I'm more the size of a woman and you would never think to put a woman in that boot.
HOWEVER, in saying all of this, part of the reason I went three piece was because of the replaceable tongues. I have recently drilled ~20 very small holes in my medium flex tongues. I'm working to soften the tongue and get me something more like a 75-80 flex boot. But with the lateral rigidity and other bells and whistles of a Krypton Pro. I'll be posting up some photos and results once I have more info. If I screw up the tongues, I can always order a second replacement set.
So long story short, I do think height and weight play a roll in the flex a skier should have in their boot. Not just skiing ability. If you don't have the weight/leverage to properly flex the boot indoors on solid ground, you're sure as hell not going to do it on a 40 degree slope in 20 degree temps... Doesn't matter how good your technique is. Meaning you'll be in the backseat.