Which is the strongest 6 by 6 post?

[Haleiwa]

Cut from a 9 inch log with the heart down the center

Cut beside the heart from a larger log

Most cedar I have available in northern New York is no larger than 18 inches.

[Mad Professor]

Hard to get a 6" X 6" from a 9" log.

I was always told to box the heart.

If you don't box the heart the beam will tend to warp and check, unless it's from a really huge log and you get away from the heart.

[Ianab]

Smaller logs, go with heart centred. 

Technically you can get a 6x6 from a 9" log, but in reality you will likely have some wane and sapwood on the small end, as logs have this annoying tendency to not be perfectly round or straight. That may be acceptable, or plan on on 10"+, and recover some useful side boards  as you are sawing. 

Trying to get four 6x6 from smaller logs is problematic. Technically you can, but you end up with the pith on one side or edge, and that makes the post unstable. If logs are big enough, then you can get completely away from the pith. Or include it in one central post, and get 5 etc. 

Cedars are more forgiving with stability than most woods, but you want the best posts, so centre the pith. 

[Mad Professor]

yes a 6" X 6" is going to be 8.4" diagonal.

Not much wiggle for less than perfect log/bark/defects.

I'd probably start with a 12" and maybe get a small 4/4 board or two.

[alanh]

Timely post, I just got a job from a guy with a pile of eastern red cedar logs, he wants as many 8x8`s as he can get...I told him we need at least 12" at the small end so some of your 16 footers might end up as 10` posts.. he looked confused but said ok..