For BMWs with batteries in the trunk, the correct size is almost always a DIN88, including ones for E46, E39, X5. The dimensions are 353 x 175 x 175/190 mm. A battery labeled DIN100 or 60038 would also have the same dimensions.
There are plenty of choices:
Motolite Gold DIN88, 85Ah, 157 RC, P6,750
ACDelco S58515, DIN88, 85Ah, 158 RC, P6,600-P7,800
Delkor 58515, DIN88, 88Ah
Delkor 60038, DIN100/DIN60038, 100Ah, P7,500
Bosch S4, DIN100, 100Ah, P8,900
Magic Power 55815, DIN88, 85/88Ah, P6,600
FB 58815, DIN88, P5,550
Puma DIN88, 85/88Ah, P5,000
Puma DIN100, 100Ah, P5,700