One Pivot said:
this isnt road biking though, you cant just click down a gear and spin faster all the time on the trail unless you're a die hard fireroad rider or something.
Not sure what the point is of this comment. You choose your gearing beforehand just like you choose your crank length. If you can't shift at some point the penalty is the same regardless. The "leverage" of a crank arm is only one aspect of the overall gearing. Tell me, what provides more leverage, a 170 crank and a 18T cog or a 180 crank and a 17T cog? They are the same. The leverage argument is a red herring; that's what gears are for. If people didn't just slap on standard ring and cassette sizes they'd get this.
You should choose your crank length based on what suits you best, just like a roadie does and for the same reasons. You may need to adjust your gearing when you do so.
Road and tri riders care about crank length so helpful discussions can be found on their boards. SlowTwitch is good for that. To summarize what I've read there, there's a broad range of crank sizes where the power output per cycle is relatively the same but the smaller crank allows for faster cadence. From a power standpoint, you are best off reducing your crank size until you can't comfortably increase your cadence. For experienced riders this is generally smaller than 175mm. There are other concerns, though, since bicycle handling and aerodynamics are effected. I have adjustable cranks on my road bike and I've done this experiment. Even though I'm 6'2" I use smaller than 175mm. Prior to that I used 175mm and wondered I should go larger because of my size. That would have been a mistake.
If you just want your large crank assumptions validated I can't offer you anything, but if you are interested in the subject seek out some informed discussions on boards where there are experts. The question here isn't the basic mechanics (which are well understood), it's how they apply to mountain biking. Leverage isn't the answer.