After a memory stick, 8gb/16gb (max xbox supports?) in size.
Ideally needs to be Xbox compatible. My 4gb doesn't have enough space for me to carry my DLC around with me for when I use other Xbox's... and my 8gb stick the Xbox says is too slow or something.
Preferably small enough to fit in a game/dvd size box properly and it still comfortably close haha.
Let me know what you have, and how much you want :)