Crank lengths for the shorter person?

I'm persevering with building up a retro-ish bike for my wife and I'm wondering about crank arm lengths. She isn't the tallest so thinking a shorter crank would benefit her riding experience.

I've always used standard 175 arms myself being 6ft, so this is new territory for me. She's about 5ft 4 so would 170 or 165 be a better fit?

At 6ft you should be using 180mm cranks, has it bothered you not ?
At least if you have a 34"+ proper leg length.

Try 170 as you can get hold of them (it's what often came on bikes that size)

170mm is fine for her !
(I've got some for sale )

Definitely crank length needs to be match with leg length. Spa Cycles do a range of their own XD2 cranks in 160 and 165 for around £35. I have bought several over the years as the kids grew up. The quality is high, as good as midrange Shimano. IIRC they are made by Stronglight.

Cheers guys. I've a set of 170s I can use so will fit those when the time comes.

As for my own leg length, pretty sure I'm a 32" not 34 or greater. When you say has it bothered me, what do you mean? Aside from numerous cartilage operations that I can't really blame cycling for, I can't say I've noticed anything odd.

Too long is definitely worse than too short. Did a bike tour with 175s (I normally ride 170s) and I had knee pain throughout.

Here is a starting point...
http://www.myra-simon.com/bike/cranks.html

It's about leg length rather than height.

Am 6.1" and my 20" Xizang has 180mm cranks, I have a 33" inside leg

Well on a good day I'm 6ft, depends on when my last visit to the chiropractors was. Generally I am 5ft 11 ish.

Back to someone 5ft 4 who has to have trousers shortened to fit when buying off the peg in her size. I can start with 170mm arms and see how they go. As someone who hasn't ridden for many years she won't know what's right and wrong, so what's the best way to tell?

Crank length is a bit of a red herring.
As long as they aren't too long (leading to you bending your knee too much at tdc) any length will work fine.
So saying that a 6 footer "needs" 180s is bullocks.
They might be able to use them, but there's no advantage to longer. Except in your head.