just found this info on germancarzone.

Boot space?
570L for X6, 620L for X5 (5 seats), 480L for X3, 500L for 7er, 520L for 5er sedan, 500L for 5er Touring, 450L for 6er coupe, 460L 3er sedan, 460L 3er Touring.
So, second largest - just after X5. I find that very sufficient ... except you need a vehicle for transporting fridges & ovens, or other large boxy things.