What crank arm length?

Good question.

I think it has more to do with  your physical size, larger folks longer cranks.

Google "Bicycle crank arm length", ton of hits...

So your going compact because your wimpy on hills.  So why change crank arms too?

OK, but I thought the term "compact crank" meant changing to smaller chain rings on the front.

Eg. changing from 53/39 to 50/34. 

Why are you changing crank arms again?

They come with the crank?  Also, I'm going from an FSA to a Shimano crank - not sure if the FSA arms would fit the Shimano crank?  Definitely wimpy on hills, though. 

Aren't we all!  When I asked why changing the arms, I was referring to the lengths.  In reading some of the google hits if your knees are not bothering you it seems that the crank arm length is ok so keep the same (being general here).  I know the LBS gets into the CA length on fitting sessions.  Power meter I guess whould be the ideal tool to know if an CA length is good or not.

Generally when you buy a new bike, the cranks are sized based on the frame size.   A smaller bike will have a 170 and a larger a 175.   Unless you have a specific reason to change, I wouldn't.   5mm is pretty small, but enough that it might make something hurt after a long ride.   Your crank arms should have a small stamp on the backside that says what they are.  Just order the same thing.