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Your question doesn't make a whole lot of sense without any restrictions on dimensions of the final USB flash device and the technology node you using for the flash chips themselves. It also doesn't account for whether or not a MCM is implemented (multiple bare dies mounted on a single substrate).
There isn't any practical limits besides cost, size, and the host's OS support.
A 2 TB USB SSD looks just like an old 4GB USB flash dongle to a PC's OS (just one has a lot more space than the other). Packaging wise the 2TB SSD is a lot bigger than the dongle.
The practical limit is probably more what the host OS can support. I'm sure a 100 PB dongle could be made but you wouldn't want to carry it around in your pocket and I doubt your OS would know what to do with it.