Thanks Einstein, "You select the spindle length that the CRANK requires."
To the legit reply , I can't measure the existing spindle as the drive spindle side is attached to the crank.
I found the solution, I know that the existing spindle has a chainline of 46 mm, so I measured from the middle of the bottom bracket to the end of the spindle on the left, added them together and got my answer. QED.