hi
when you lift up to find the wheel and it's not there, there should be a carpet that you can lift up to reveal another part. It could be there. If not, it needs to be at least a 16" one because of the size of the brakes. Make sure it's from a 3 series cos the 5 series is different.
I would give the stealers (BMW) a buzz first. they are usually very helpful just to confirm this info is correct. A couple of people have been saying 17" but a lot are saying 16". 15" wont' go over the larger brakes of the 330.
when you are definite, there are a few on ebay