Crank Arm length

My wife has similar measurements.  We got her a Javelin this spring and it has a compact crankset (50/34 w/ 110 bcd)  I was able to find some "Bulletproof" cranks that work with her Campy 9-speed drivetrain (square-taper BB) in lengths below 170.  I think I got her 160's. 

Maybe. It sounds like you're more pleased with the road bike, "I noticed that my pedal strokes felt smoother and more efficient." If this is true, then maybe you've answerd the question.

You can find only arms and swap chainrings, but typically, it's more economical to buy everything together. Understand that you can't buy compact arms and bring over existing standard crank rings. Or vice versa. If you want to change that, buy a whole set-up.

Coming from Tiagra, you might have incompatilbility issues with the bottom bracket (the part in the frame which holds and allows the cranks to spin). A Tiagra BB might not work with a new, say FSA crankset. These change rapidly from year to year, so be sure of what you're marrying together.

If you're not upgrading to carbon fiber, you can get deals on Shimano 105 often for <$100. I saw Dura Ace cranksets (but 53/42 rings?) on chainlove.com yesterday for the low, low price of $279. FSA Gossamer is a solid choice and usually relatively inexpensive.