As Larry says, should be 12v if a Light 15. A comparable French Legere would be 6v (if not converted to 12).
I have an Exide EA654 in my 12v conversion Legere which is 232 (L) x 173 (W) x 225mm (H) and it’s about the biggest that will fit in the battery box and the best CCA (cold cranking amps) to help spin the starter that I could find.
David Faulkner