For the OP.
At 6'10" you will almost certainly need a custom frame as well as some custom components (cranks).
If you are anywhere close to being of normal proportions is terms of leg length vs. torso length, you will need cranks that are at least 180mm. There is a good chance your pedalling mechanics would benefit from crnaks as long as 200mm.
Since most bicycle manufacturers do not take cranks of that length into account when designing their frames, pedalling in the corners and foot overlap with the front wheel will be concerns that needs to be addresses.
Lennard Zinn is a renowned builder of over-sized bicycles.
http://zinncycles.com/Zinn/?p=5
I am 6'7" and currently weight about 205. My racing weight was 188 - 192 when a collegiate and USCF rider. I have a degree in biomechanics and have spent a lot of time studting the specific needs of tall cyclists.
I raced back in the 80's on a 63cm Cannondale with a 140 mm stem and 175mm cranks. I retired from crits and traded that bike for a custom-built steel frame of 65cm, still had the 140mm stem and switched to 180mm cranks. I felt much better on the longer cranks and produced more power, though they were a little tricky in the corners, but since I was focusing on road races it was a good switch.
As for the advantage of increased crank arm length:
The equation of force x force arm length = resistance x resitance arm length is important.
In cycling the force is the application of muscular contractile strength to the pedal. The force arm is the length of the crank. Resistance is determined by speed, wind, frinction in the bicycle, and rolling resistance of the tires. The resistance arm is complicated to fully calculate, but can be considered as a constant. For all practical purposes, the right side of the equation can be considered as a constant when calculating the benefits of longer cranks.
A longer force arm (crank) gives a mechanical advantage. This principle is the same for the rationalle of using crowbars are used to lift manhole covers. A crank of 180mm is 2.8% longer than a crank of 175mm. Thus, there is a 2.8% increase in the amount of force applied to the chain with the same amount of exertion from the rider. A crank of 190mm yields an increase of 7.8% when compared to a crank of 175mm.
If your legs are of a length where you can pedal a 190 with appropriate angle measurements for your knees, you will attain a huge advantage (almost 8%) in power from using the longer cranks when compared to the 175's.
Us big guys have a lot of disadvantages in cycling. We have more wind resistance to push through, have to lug a lot more weight up climbs, have to get more mass moving when accelerating out of corners, and have to pay more for custom equipment designed to fit our bodies. We owe it to ourselves to seek an advantage that can help to reduce some of those disadvantages.
I used to line up for a crits next to guys who were about 5'6" quite frequently. I noticed they were on 170mm cranks and I was on 175 mm cranks. That is a difference of only 2.9%. However, there is a 16.5% difference in our heights. It just didn't make sense to me. I realize there is more to determing crank length than the height of the rider, but for rough estimates sake...
I had the chance to test ride a Zinn bicycle that was built with 200mm custom cranks. I went for only about a fifteen minute ride, but it felt wonderful!
To summarize. If you want to get the most enjoyment out of your riding, invest in some equipment that is custom built for your massive frame. The stuff isn't inexpensive, so take some time to consult with as many expert frame disigners as you can. Consider longer cranks. Zinn's company can fabricate custom cranks for you and create a bike with your specific needs in mind.
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