The specs on the official website make no mention of any USB pen drive capacity limit so I'd take that to mean there isn't one.
I've yet to come across a device that has a capacity limit where a pen drive drive is concerned. Memory cards yes, but not pen drives. Anyway, it's only a pen drive. It's hardly going to be an expensive waste of money if a 128 GB one is too much for it, but I'd be very surprised if it was.