I don't think USB size really matters, more to do with how much files you have and if the USB is good in terms of performance. Probably won't have difference between USB 2.0 and 3.0 as our car is most likely 2.0 only... but 3.0 USB might have better performance still when compared to 2.0 flash drives.
That first bit isn't true. Mazda were quite specific about the USB drive size limitation, and from an IT perspective the host system (in this case the Mazda media system) always knows what size of drive has been plugged in irrespective of the amount of songs on it. This is because the USB drive informs the system what capacity it is before the files begin to be read. I would therefore stick to an 8GB drive, and avoid any potential issues, because ultimately in my experience the larger drives eventually kill the module. Also 8GB drives are dirt cheap, so why bother taking the risk?
In terms of USB2.0 vs USB3.0, the system itself is USB2.0, but since drives are backwardly compatible a USB3.0 drive (which I use) will always ensure that you are getting the maximum possible performance out of their USB2.0 interface.