The length of bb spindle you need depends on what cranks you get. Two cranks on identical length spindles may put the chainring in different places.

However long a spindle you wind up getting, make sure your bb is english threaded with a 68mm body. That's what your frame's bb shell is set up for.

Also, if you are getting square taper cranks, it's good to make sure that the taper on your spindle is correct*. There are two standards, ISO and JIS. Which one you want will depend on what cranks you get. Most cheaper cranks are JIS but check and make sure.

*You can mix tapers but it affects chainline and you have to compensate for that by changing bb spindle length---it's slightly easier to just get the correct taper.