You sure about that? Check out the trunk photo in this thread:
http://www.xoutpost.com/bmw-sav-items...ull+size+spare

Looks like he fit a front 20" in there! Amazing because I couldn't fit a 19" rear when I needed to.

In any case, as far as I know all the X5's came stock with 17" full size spares with the exception of the 'is models which have space saver spares that are 19". The advantage of the 'is models is that since the wheel is so much smaller you have a nice storage area under the load floor. If you don't mind losing that area, you'll have to strip out all the space saver mounting parts and the trunk floor to stick in a full size 20" front in there like that guy did.

The largest full size space I've seen fit without any modification is 18".