How do I know which scale length to use?

The taper of the string should occur between the tuning post and the nut. A simple method to determine this length is to mark the existing string on your bass at the nut position and then, after removing the string, measure the length between the string-side of the ball end and the nut mark.

Elixir® Strings Long Scale - 34" to 36"

Elixir® Strings X-Long Scale - 36" to 38"