I believe the batteries for the X5s, is either a T8 or H8 DIN size. In terms of dimensions, I believe it is:

Length 13 16/15"
Width 6 15/16"
Height 7 1/2"
Height 6 7/8" (T8)
I would double check with your local BMW dealer or any dealer of car batteries. THe only physical difference between the H8 and T8 is that the H8 is taller.

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2006 X5 3.0L - Schwarz Black/Truffle Brown
Stock
But I keep it clean