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.
I bought a 128 GB USB Thumb drive, and yes it works great in the Rocu 3.
I loaded the drive with mp4 files that were converted from HD, mp3 files, using Magix 4.
Rocu tech support, (on-line) was no help. They said the limit was 2 GB!
I told the guy He had that number confused with the micro SD flash drive specs.