I'm using a 32GB USB stick in my DSport and have no problems with it. That said, I only have around 1,000 tracks on it so it's by no means full. The only complaint I have is that the eMyWay doesn't seem to identify tracks saved at 320kbps...
Coops - I have two 16gb memory sticks runing.Reason is that one with about 12gb of music seemed to work okay, but I noticed it had 'lost' dozens of albums, which I verified were okay on my computer.
I found that splitting onto 2 sticks with about 6gb on each was fine.
Interestingly the new Grand Picasso has a built in hard drive on exclusive with 8gb!! So this would seem to be an arbitrary limit that Citroen have set.