Is there a reason why a road bike would have 170 crank arms over 175s. Dose the frame size have anything to do with it? I have a large steel frame ,59cm? with 170s. It seems strange to me.

There's probably a general tendency for tall people (who ride big frames) to like longer crank arms, but it's far from universal - i.e. there are some tall people who still prefer fairly short cranks. And there are also lots of us who don't see much of a difference. I have two 60cm bikes, one with 170mm cranks and the other with 175s, and I can't really notice any difference. I do notice a little difference riding my tandem which has 165mm cranks, but even that is pretty minor and certainly it wasn't enough to consider switching them to another size.

https://zinncycles.com/Zinn/index.php.../custom-cranks

He describes the sizing issues. About 21% of inseam. Not written in stone, but something to consider.