In my opinion, when buying USB-based storage products (flash memory, external hard drives, etc.), the most important factor is not the size; but the supported data transfer rate. Transfer rate does not matter with keyboards or mice. The USB specification has defined four transfer rate classes. They are:
|1.0||Low speed||1.5 Mbit/s|
|1.1||Full speed||12 Mbit/s|
The numbers don't lie. Sure, they're theoretical, but they're a good guideline when buying USB products. If you need portable storage, get at least Hi-speed or SuperSpeed (USB 3.0) if your system supports it. Anything below that isn't worth it.