At 5'9-5'10 with medium sized frames, what size cranks should i be riding? I do mainly XC, some AM riding. No downhill or freeride.
No reason to have to deviate from the "standard"
If you really like to spin, you could get some 170, but I wouldn't recommend it.
Pardon my naievity, but what do you mean by spin????? Why wouldnt you recommend the 170's?
spinning your legs fast (ie 100+ rpm) rather than putting effort (grinding/mashing) into each pedal stroke.
If you like to pedal fast shorter cranks are easier to do that with.
If you like to pedal slow then longer cranks give more leverage.
I would assume most of your riding has been on 175s. Do you have any issues with them? If not I'd stick with it.
I like to spin and prefer 172.5 MTB cranks at 6' with a 33" pants inseam.
And I swap crank lengths (between 165 and 175) on my road bike depending on the ride route or race course.