Smaller crank arms for cross
 

I was about to pull the trigger on new 175mm cranks; But am now considering going w/ smaller cranks 172.5 after reading this: http://sheldonbrown.com/cranks.html

Looking at OEM cross bikes, it seems that the cranks are 2.5mm shorter than road bike equals. Is this for toe clearancein the turns, or for easier spinning up hills.

What is the general consensus?

Do the shorter CX cranks feel weird coming from longer on a road bike? I am 6'-4" tall, and until now never considered shorter cranks.

What is your inseam? My initial thought is at 6'4" you are going to want 175s. I am 6'1" with a 34" inseam and went from 172.5s to 175s. It doesn't make a big difference but longer cranks give you more leverage for steep climbs and technical terrain.

You will gain a little clearance with the shorter cranks but ultimately bb position will dictate pedal strike and toe overlap. I've got no problems spinning 175s and 180s (100rpm average according to my garmin) and my legs are likely shooter than yours.

Most modern CX frames don't have a high BB like older CX frames. Clipless pedals made it unnecessary. Granted the larger tires will raise up the whole bike a bit. I'd just stick to what you ride on the road or can get a good deal on.

Your frame is BB30 right? Are you using an adapter to get shimano cranks in there?

I'm 6'6" with a 36" inseam and I ride 175mm on my distance, commuter, brevet, and mountain bikes. I put a 172.5mm crank on my CX bike this year, swapping out from the 175mm I had last year.
If I was blindfolded and told to pedal one, then the other, I don't think I could tell you the difference.

Yes, FSA BB30 to 68mm threaded adapter.

My inseam is 35"

If you are buying a new crank, why not get a BB30 crank?