Crank arm length is an entirely anatomical consideration. If you really want to build the bike right, you absolutely have to get someone to fit you.

www.competitivecyclist.com has a fit calculator that I've used in the past and find it to be very good. It even predicted things for me such as using a non-setback seatpost (even with road race geometry, much less TT/Tri geometry) that a professional fitter would really have to know what they're doing to suggest.

It's my understanding that crank length is very closely related to your femur length, but it's slightly more complicated than that because of the relationship between crank length and [virtual] seatpost angle. In the end, what you want to arrive at is a geometry where you pass the classic "plumb line" test for a straight vertical line between your kneecap and pedal spindle (though a TT/Tri position often puts your kneecap as much as 2cm or so in front of the crank spindle. When I nose ride for an aggressive position my kneecap is way in front of my toes). There are actually bike CAD programs for the computer that can model this for you, but that's why I suggest the competitive cyclist calculator. Google "Plumb line kneecap pedal" or something like that and I'm sure you'll find all the info you need.

As far as muscle utilization and crank arm length, I'm actually not sure what the consensus is. One thing I've become very aware of since using a power meter, though, is that power is a result of foot force and foot speed. If you have a long lever arm, you'll get more force, but your foot will also be traveling further, so I believe it all comes out in the wash. That's why I believe crank arm length is more based on geometry.

One last thought is pedal strikes on the ground. For triathlon, you're not likely to have to pedal through corners as aggressively as, say, a criterium rider would, but it's still something to think about. On my fixed gear I use 165mm cranks, but I have a 650c front wheel and I get pedal strikes if I take turns too fast (because I obviously can't coast with the inside foot up). Let me tell you, that is NOT a fun experience. I think I've only had pedal strikes on my road bike once or twice and they're equally unsettling.

I switched my roadbike cranks from 170s to 180s and really liked the change (I'm 6'4" with 36" inseams). Leonard Zinn (www.zinncycles.com) suggested longer (like 190s) but I felt that might be putting too much strain on my hips and knees initially. For a tall person it should be better to have longer cranks - you won't be gaining any real "advantage", but for the shape of your body you might be able to use more of your leg (and tire less easily) by using a longer crank. I think you must give your body some time to adjust. Also, I have had to train myself to spin a little more since with the longer cranks it's easier to push a bigger gear; but that isn't necessarily what you want to do to your body.

One other note: when I was shopping for a tri bike, the LBS dissuaded me from longer cranks (they felt 175s would be fine for me) due to the more forward position for a tri bike - that with longer cranks it would be too hard to get the feet over the pedals enough. I'm not sure if I agree with that rationale, but I stuck with the 175s on the tribike. To date, I'm still not as powerful on it as I am on my road bike with the 180s. But, I might be saving myself a little better for running with the shorter cranks.