There is a nerdy way if you have to choose a starting spindle length completely in the dark. It's not perfect but usually gets close and may nail it.
Arrive at a premise for JIS vs ISO. Almost all Asian cranks are JIS with the exception of a few higher end track cranks.
Measure the width of the square taper spindle at its tip. I don't have those numbers in front of me. Mark the taper bore about 2mm out from where it hits that inner measurement. The 2mm is to account for the press fit. It's a fudge but is about right.
With a vernier caliper and squinting, get a measurement for the lateral distance your mark falls past the centerline of the chainring(s), ie the tooth centerline for 1x or 3x or the midpoint for 2x. Call that distance X. If somehow it wasn't past that centerline, it would become a negative number.
Arrive at a number for target chainline. Call that C.
You now have everything you need for taking a guess at the spindle length math. Call spindle length S. You're now solving S= 2(C + X). (This assume a modern crank that wants symmetrical spindle.) Again it's not perfect, but it's a way to get to a starting point if you must order a BB with no other information.