Not all cranks are the same yeah ?
If you want a raceface turbine crank on there it will require an axle of a certain length
If you want a Shimano XTR crank on there it will require an axle of a certain length
If you want a middleburn crank on there it will require an axle of a certain length which is a different length to the turbine one and the XTR one.Or they might be the same :? Point is you decide what crank you want to put on then ask us if we know which axle suits it. Given the VAST depth of knowledge on here its highly likely someone will know the info off the bat.
Its the crank itself that determines the length of the axle. All cranks are different so you can assume that its likely all axle lengths are different, this isnt completely the truth however. Many things are very similar so you find many things are interchangeable.
You can look deeper into this concerning chainline,frame etc but for the vast majority you dont need to,its such and such a crank and therefore its such and such an axle length.
I myself had a bit of a job finding the correct length for a pair of early American AC components cranks. There was nothing i could find to easily get the makers recommended axle length and in the end it was someone posting a picture of the cranks from a company flyer that had the axle length info on it.
I hope this has actually come across clearer to you and not made things worse
I take it you intend to grease the axle taper ?