What is my bottom bracket spindle length?
The only measurement for a bottom bracket you can do with the bottom bracket still on the bike is to do the shell width, which is the width of the frame where the bottom bracket installs.

In order to measure the spindle length, you need to remove the cranks and then measure the spindle length. There is no guarantee that the spindle is symmetric or the width of the cranks on both sides is the same. In any case, you'll need to pull off the cranks to replace them.