I don't think size would be much of an issue. Random googling finds USB2512[1] which comes in a 6mm x 6mm QFN package. It needs a crystal and a few caps. There'

I don't think size would be much of an issue. Random googling finds USB2512[1] which comes in a 6mm x 6mm QFN package. It needs a crystal and a few caps. There's probably an equally tiny flash + USB SoC around to bolt onto it. The hard part would be the board layout (USB2 requires some line impedance matching, afaik) and the actual packaging. I reckon you could (conservatively) fit the necessary circuitry into 1 or 2 microSD sized cuboids, maybe one above the plug/socket, and one below. Maybe even less if you eschew some of the spec and only support daisy-chaining your own devices. [1] http://www.smsc.com/index.php?tid=295&pid=23&cid=&tab=1 http://www.smsc.com/index.php?tid=295&pid=23&cid=&#3...