at the factory they may use smaller or bigger batteries in order to save on the cost - why stick a big battery into a car that has less demands for power, like no heated seats, no rear blower, manual A/C controls... it would make sense to have one size for all models but from the cost savings point, it is better to use various size batteries... therefore, in the battery tub there are 3 tie-down points, to take the various sizes... while you can't really go too small on a fully loaded "is" version, you can always go a size or two too big from the original version, as it might make more sense on the individual owner level to go "max-size" on a normal E53...

look for H8 battery with a vent, and the "AGM" thingy... can't go wrong with that size... there is one size bigger but i think that one will not fit into the tub, as H8 is taking the last slot...